#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    ll a, b, c;
    cin >> a >> b >> c;

    ll t = a * 60 - b;
    if (t <= 0) {
        cout << -1 << endl;
        exit(0);
    }

    cout << (c * 3600 + t - 1) / t << endl;

    return 0;
}