#include<iostream>

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    long long a, b, c;
    cin >> a >> b >> c;
    a = a * 60;
    c = c * 60 * 60;
    if (a == b) {
        cout << -1 << endl;
    } else {
        long long x = (c + (a - b - 1)) / (a - b);
        if (x < 0) cout << -1 << endl;
        else {
            cout << x << endl;
        }
    }
}