#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;

	cout << (double)(abs(px-qx) + abs(py-qy))/2 << endl;
	
	return 0;
}