#include <stdio.h>
#include <math.h>

int main() {
    double a,b,c;
    scanf("%lf %lf %lf", &a, &b, &c);

    a*=60;
    c*=3600;

    double x = c / (a-b);
    long ans;
    if (x < 0) {
        ans = -1;
    }
    else {
        ans = (long) ceil(x);
    }
    printf("%ld\n", ans);
    return 0;
}