#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(){
  Int px,py,qx,qy;
  cin>>px>>py>>qx>>qy;
  double ans=0;
  ans+=abs(px-qx)/2.;
  ans+=abs(py-qy)/2.;
  //if((abs(px-qx)&1)&&(abs(py-qy)&1)) ans+=0.5;
  cout<<ans<<endl;
  return 0;
}