#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;

#define ll long long
#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	int x1,x2,y1,y2;
	cin >> x1 >> y1;
	cin >> x2 >> y2;
	int ans = abs(x1 - x2) + abs(y1 - y2);
	if(ans % 2 == 0){
		cout << ans / 2 << endl;
	}else{
		cout << ans / 2 << ".5" << endl;
	}
	return 0;
}