#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007

using namespace std;


int main(void) {
	int px, py, qx, qy;
	double math;
	cin >> px >> py >> qx >> qy;
	math = (abs(px - qx) + abs(py - qy)) / 2;
	cout << math << endl;
	return 0;
}