#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
using namespace std;
#define Would
#define you

int ans;
int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	int mon = abs(c - a) + abs(d - b);
	cout << mon / 2;
	if (mon % 2 != 0) { cout << ".5" << endl; }
	cout << endl;
}