#include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;

int main(){
	int px, py, qx, qy;
	cin >> px >> py >> qx >> qy;
	printf("%.2f\n", (abs(px-qx)+abs(py-qy))/2.0);
	return 0;
}