#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>
using namespace std;

int main(){

	int px,py;
	cin >> px >> py;
	int qx,qy;
	cin >> qx >> qy;

	double ans = (double)(abs(px-qx) + abs(py-qy))/2;

	printf("%.20lf\n",ans);
	
	return 0;
}