#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>

#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
#define INF 1000000000

using namespace std;


int main(void) {
	int px, py, qx, qy;
	double x, y;

	cin >> px >> py >> qx >> qy;
	x = abs(px - qx);
	y = abs(py - qy);
	
	printf("%.5lf\n", (x + y) / 2.0);

	return 0;
}