#include <stdio.h>

int main(void)
{
    long A, B, C;
    scanf("%ld%ld%ld", &A, &B, &C);
    A *= 60;
    C *= 3600;
    if (C / (A - B) > 0) printf("%ld", C / (A - B) + 1);
    else puts("-1");
    return 0;
}