#include<iostream>
#include<math.h>
#include<string.h>
#include<vector>
#include<algorithm>
#include<iomanip>
#include<deque>
#include<map>
#include<stdio.h>
using namespace std;

int main(){
	int px,py;
	cin >> px >> py;
	int qx,qy;
	cin >> qx >> qy;
	double ans=abs(px-qx)+abs(py-qy);
	ans/=2;
	cout << ans << endl;
	return 0;
}