#include<iostream>
#include <math.h>
#include <string>
using namespace std;

int main(){
	double a;
	double b;
	double c;
	double d;
	cin >> a;
	cin >> b;
	cin >> c;
	cin >> d;
	cout << fabs(a - c) / 2 + fabs(b - d) / 2 << endl;
	return 0;
}