#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;
  cout << (c/(a-b)+1>0?c/(a-b)+1:-1) << endl;
  return 0;
}