#include<bits/stdc++.h>

using namespace std;

int main(){
  long long int 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;
  }
}