//================================= // Created on: 2018/09/28 22:42:53 //================================= #include 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; }