diff --git a/du5/program.c b/du5/program.c
index ed46bd8..9626941 100644
--- a/du5/program.c
+++ b/du5/program.c
@@ -20,7 +20,9 @@ int lineCount = 0;
 int indexLine = 0;
 int nodeCount = 0;
 int answerCount = 0;
-int invalidIndex = -1;
+
+//pomocna konstanta
+const int invalidIndex = -1;
 
 // vytvorii binarny strom na zakl. vstupu
 //** VRATI -> Bud cislo > 0 (Platny Koren) ALEBO stav '-1' (Neplatny index korenu, cize koniec stromu)