#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main() {
    ll a, b, c;
    cin>>a>>b>>c;
    a *= 60;
    c *= 3600;
    if (a <= b) puts("-1");
    else cout << (c+a-b-1) / (a-b) << endl;
    return 0;
}