#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0 ; i < (n) ; i++)

#define int long long


signed main(){
	double x,y,X,Y;
	cin >> X >> Y >> x >> y;
	printf("%.5lf\n",0.5 * (abs(X-x) + abs(Y-y)));
}