#include<bits/stdc++.h>
using namespace std;

int main(){
	int px, py, qx, qy;
	cin >> px >> py >> qx >> qy;
	int x = max(px, qx) - min(px, qx);
	int y = max(py, qy) - min(py, qy);
	double ans = (double)(x + y) / 2;
	printf("%f\n", ans);
}