#include <iostream>
using namespace std;

int main() {
    double A, B, C;
    cin >> A >> B >> C;
    A *= 60;
    C *= 3600;
    if (B > A) cout << -1;
    else {
        if ((long)(C / (A - B)) == C / (A - B)) cout << (long)(C / (A - B));
        else cout << (long)(C / (A - B) + 1);
    }
}