#include <bits/stdc++.h>

using namespace std;

using int64 = long long;

int main() {
  int64 A, B, C;
  cin >> A >> B >> C;
  A *= 60;
  C *= 3600;
  if(A <= B) {
    cout << -1 << endl;
  } else {
    cout << (C + (A - B) - 1) / (A - B) << endl;
  }
}