//=================================
// Created on: 2018/09/28 22:42:53
//=================================
#include <bits/stdc++.h>
using ll = long long;
int main()
{
    ll A, B, C;
    std::cin >> A >> B >> C;
    A *= 60, C *= 3600;
    std::cout << (A <= B ? -1 : (C + A - B - 1) / (A - B)) << std::endl;
    return 0;
}