#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <string>
#include <cstdlib>
#include <stack>
#include <queue>
#include <cmath>

#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define repp(i,j,k) for(int i=j;i>=k;i--)
#define Sort(x) sort(x.begin(),x.end()); 
#define ll long long
int mod=1e9+7;
const int INF = 1e9;

using namespace std;


int main(void) {
	double x, y, xx, yy;
	cin >> x >> y >> xx >> yy;
	printf("%f\n", abs(x - xx) / 2 + abs(y - yy) / 2);


	return 0;
}