pvjc20/du1/program.c

#include <stdio.h>

int main(){

    char ch;
    int num = 'a' - 'A';
    scanf("%c", &ch);
    if(ch == EOF || ch == -1){
        return 0;
    }
    if(ch >= 'A' && ch <= 'Z'){
        ch += num;
    }
    else if(ch >= 'a' && ch <= 'z'){
        ch -= num;
    }
    printf("%c", ch);
    
}