#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>

//#include<bits/stdc++.h>



using namespace std;




int main() {

	double a,b;
	cin>>a>>b;
	double c,d;
	cin>>c>>d;
	double p=abs(a-c)+abs(b-d);

	
	cout<< fixed << setprecision(1)<<p/2<<endl;

	return 0;
}