#include <bits/stdc++.h>
using namespace std;
const long long MOD=1e9+7;
int main(){
  long long a,b,c;
  cin>>a>>b>>c;
  a*=60;
  c*=3600;
  if(a-b<=0){
    cout << -1 << endl;
    return 0;
  }
  cout << (c+a-b-1)/(a-b) << endl;
  return 0;
}