#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
typedef long long ll;
#define int long long

signed main(){
  ll a,b,c;
  cin>>a>>b>>c;
  if(60*a<=b){
    cout<<-1<<endl;
    return 0;
  }
  cout<<setprecision(18)<<ceil((double)c*60.0*60.0/(double)(60.0*a-b))<<endl;


  return 0;
}