#include <stdio.h>

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