#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  double px,py,qx,qy;
  cin>>px>>py>>qx>>qy;
  double ans=0;
  ans+=abs(px-qx)/2;
  ans+=abs(py-qy)/2;
  cout<<ans<<endl;
  return 0;
}