#include <stdio.h>

int main(){
    int A, B, C;
    int X = 0;
    scanf("%d %d %d", &A, &B, &C);
    A *= 60;
    C *= 3600;
    if( A <= B ){
        X = -1;
    }else{
        X = C / (A-B) + 1;
        if( C % (A-B) == 0 ) X--;
    }
    printf("%d", X);
}