#include <bits/stdc++.h>
using namespace std;

using ld=long double;

signed main(){
  ld xa,ya,xb,yb;cin>>xa>>ya>>xb>>yb;
  xa*=-1;
  ld ans=ya+(yb-ya)*-xa/(xb-xa);
  cout<<fixed<<setprecision(12)<<ans<<endl;
}