diff --git a/du4/program b/du4/program
new file mode 100755
index 0000000..9bec58c
Binary files /dev/null and b/du4/program differ
diff --git a/du4/program.c b/du4/program.c
new file mode 100644
index 0000000..e349f85
--- /dev/null
+++ b/du4/program.c
@@ -0,0 +1,239 @@
+#include<stdio.h>
+#include<stdlib.h>
+#include<string.h>
+#include<stdbool.h>
+#include<ctype.h>
+
+#define SCAN_FIRST_NUMBER 0
+#define SCAN_FIRST_OPERATOR 1
+#define SCAN_SECOND_NUMBER 2
+#define SCAN_SECOND_OPERATOR 3
+#define SCAN_RESULT 4
+// #define SCAN_END 5
+
+char* scanValue(char *s, float *output);
+char* scanOperator(char *s, char *output);
+
+// bool isDigit(char c);
+bool isOperator(char c);
+// bool isFloatingPoint(char c);
+// bool isWhitespace(char c);
+bool isNewline(char c);
+bool evaluate(float a, float b, float c, char op);
+
+void error();
+
+int main(){
+    char buffer[2048];
+    // char str[512];
+    
+    float a;
+    float b;
+    float c;
+    char operator;
+    
+    char *retVal = fgets (buffer, sizeof(buffer), stdin);
+    
+    if (retVal == 0 || isNewline(buffer[0]) == true) {
+        puts("KONIEC");
+
+        return 0;
+    }
+
+    int l = strlen(buffer);
+
+    if (isNewline(buffer[l - 1]) == false) {
+        error();
+
+        return 0;
+    }
+
+    int scanStatus = SCAN_FIRST_NUMBER;
+    char *currentChar = buffer;
+    char *endChar = buffer + l - 1;
+
+    // for (char *i = 0; i < l; i++) {
+    //     // int forward = 0;
+        
+    //     char currentChar = buffer[i];
+
+    //     if (i == l - 1 && isNewline(currentChar) == false) {
+
+    //     }
+    // }
+
+    while (currentChar < endChar) {
+        if (scanStatus == SCAN_FIRST_NUMBER) {
+            retVal = scanValue(currentChar, &a);
+        } else if (scanStatus == SCAN_FIRST_OPERATOR) {
+            retVal = scanOperator(currentChar, &operator);
+        } else if (scanStatus == SCAN_SECOND_NUMBER) {
+            retVal = scanValue(currentChar, &b);
+        } else if (scanStatus == SCAN_SECOND_OPERATOR) {
+            char eqOp;
+            
+            retVal = scanOperator(currentChar, &eqOp);
+
+            if (eqOp != '=') {
+                retVal = 0;
+            }
+        } else if (scanStatus == SCAN_RESULT) {
+            retVal = scanValue(currentChar, &c);
+        } else {
+            error();
+
+            break;
+        }
+
+        if (retVal == 0) {
+            error();
+
+            break;
+        } else if (scanStatus == SCAN_RESULT) {
+            bool validResult = evaluate(a, b, c, operator);
+
+            if (validResult == true) {
+                puts("OK");
+            } else {
+                puts("ZLE");
+            }
+
+            break;
+        }
+
+        currentChar = retVal;
+        scanStatus++;
+    }
+
+    return 0;
+}
+
+char* scanValue(char *s, float *output) {
+    char *retVal = 0;
+
+    *output = strtof(s, &retVal);
+
+    if (retVal == 0 || retVal == s || ((*output) == 0 && *(retVal - 1) != '0')) {
+        return 0;
+    }
+
+    return retVal;
+    
+    // int outPosition = 0;
+    // int l = strlen(s);
+    // bool hasPoint = false;
+
+    // for (int i = 0; i < l - 1; i++) {
+    //     char c = s[i];
+    //     char cNext = s[i+1];
+    //     bool isError = (outPosition == 0 && isDigit(c) == false)
+    //         || (outPosition > 0 && (
+    //             (hasPoint == true && isFloatingPoint(c))
+    //             || (hasPoint == false && isFloatingPoint(c) == true && isDigit(cNext) == false)
+    //             || (isDigit(c) == true && (
+    //                 (hasPoint == true && isOperator(cNext) == false && isWhitespace(cNext) == false && isNewline(cNext) == false)
+    //                 || (hasPoint == false && isOperator(cNext) == false && isWhitespace(cNext) == false && isNewline(cNext) == false && isFloatingPoint(cNext) == false))
+    //             )
+    //             || (hasPoint == true && isDigit(c) == false)
+    //             || (hasPoint == false && isFloatingPoint(c) == false && isDigit(c) == false)
+    //         ));
+
+    //     if (isError == true) {
+    //         return 0;
+    //     }
+
+    //     if (isFloatingPoint(c)) {
+    //         hasPoint = true;
+    //         outPosition++;
+            
+    //         continue;
+    //     }
+
+    //     bool endsHere = isOperator(cNext) == true || isWhitespace(cNext) == true || isNewline(cNext) == true;
+
+    //     if (endsHere == true) {
+    //         break;
+    //     }
+
+    //     outPosition++;
+    // }
+
+    // if (outPosition > 0) {
+    //     *output = strtof(s, s + outPosition - 1);
+    // }
+
+    // return outPosition;
+}
+
+char* scanOperator(char *s, char *output) {
+    char *start = s;
+
+    while (isspace(*start) != 0) {
+        start++;
+    }
+
+    char c = *start;
+
+    if (isOperator(c) == false) {
+        return 0;
+    }
+
+    (*output) = c;
+
+    return start + 1;
+}
+
+// bool isDigit(char c) {
+//     return c >= '0' && c <= '9';
+// }
+
+bool isOperator(char c) {
+    return c == '+' || c == '-' || c == '*' || c == '/' || c == '=';
+}
+
+// bool isFloatingPoint(char c) {
+//     return c == '.' || c == ',';
+// }
+
+// bool isWhitespace(char c) {
+//     return c == ' ' || c == '\t';
+// }
+
+bool isNewline(char c) {
+    return c == '\n';
+}
+
+bool evaluate(float a, float b, float c, char op) {
+    float resultL = c - 0.001F;
+    float resultR = c + 0.001F;
+    float result;
+
+    switch (op)
+    {
+        case '+':
+            result = a + b;
+            break;
+
+        case '-':
+            result = a - b;
+            break;
+
+        case '*':
+            result = a * b;
+            break;
+
+        case '/':
+            result = a / b;
+            break;
+        
+        default:
+            error();
+            return false;
+    }
+
+    return result == c || (result >= resultL && result <= resultR);
+}
+
+void error() {
+    puts("CHYBA");
+}