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

int main(){
	cin.tie(0),ios::sync_with_stdio(false);
	int px,py,qx,qy; cin>>px>>py>>qx>>qy;
	cout<<fixed<<setprecision(10)<<(double)(abs(px-qx)+abs(py-qy))/2<<"\n"s;
}