#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;
	
	int dx=abs(px-qx);
	int dy=abs(py-qy);
	
	int dist=(dx+dy)/2;
	cout << dist;
	if((dx+dy)%2!=0){
		cout << ".5";
	}
	cout << endl;

	return 0;
}