#include<iostream>
using namespace std;
int main() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	if (a > c) {
		a -= c;
	}
	else {
		a = c - a;
	}
	if (b > d) {
		b -= d;
	}
	else {
		b = d - b;
	}
	cout << (1.0*a + b) / 2 << endl;
}