#include<iostream>
#include<cmath>
using namespace std;

int main() {
    long a, b, c;
    cin >> a; cin >> b; cin >> c;
    
    if (b >= 60 * a){
        cout << -1 << "\n";
        return 0;
    }

    long tmp;
    tmp = static_cast<long>(ceil(3600.0 * c / (60 * a - b)));

    cout << tmp << "\n";
}