#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int main() {
  ll A, B, C; cin >> A >> B >> C;
  if (A * 60 < B) {
    cout << -1 << endl;
    return 0;
  }
  ll X = A * 60 - B;
  cout << (C * 3600 + X - 1) / X << endl;
  return 0;
}