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





//-----------------------------------------------------------------
int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;

	int ans = abs(a - c) + abs(b - d);

	cout << ans / 2;
	if (ans % 2 == 1) cout << ".5";
	cout << endl;
}