#include <iostream>

using namespace std;
int main(void){
  long A, B, C;
  cin >> A >> B >> C;
  long a = 60 * A;
  long b = B;
  long c = 3600 * C;
  if(a <= b){
    cout << -1 << endl;
    return 0;
    
  }
  
  if(0 == c % (a - b)){
    cout << (c / (a - b)) << endl;
    
  }else{
    cout << (c / (a - b) + 1) << endl;;
    
  }
  return 0;
  
}