#include<bits/stdc++.h>
using namespace std;
using P = pair<int, int>;

int main() {
    long long a, b, c;
    cin >> a >> b >> c;
    a *= 60;
    c *= 3600;
    if (a <= b) {
        cout << "-1\n";
        return 0;
    }
    cout << (c + a - b - 1) / (a - b) << "\n";
    return 0;
}