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

struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
signed main(){
  double xa,ya;
  double xb,yb;
  cin>>xa>>ya>>xb>>yb;
  xa*=-1;
  cout<<ya-(yb-ya)/(xb-xa)*xa<<endl;
  return 0;
}