#include <iostream>
#include <fstream>
#include <vector>
#include <cstring>
#include <string>
#include <algorithm>
#include <iomanip>
#include <cmath>
using namespace std;

double x[2], y[2];
double f(double m) {
	return sqrt(x[0] * x[0] + (m - y[0]) * (m - y[0])) +
		sqrt(x[1] * x[1] + (m - y[1]) * (m - y[1]));
}

template<class Function>
double ternarySearch(double l, double r, Function f) {
	for(int i = 0; i < 200; i++) {
		double a = (l + l + r) / 3;
		double b = (l + r + r) / 3;
		if(f(a) > f(b)) l = a;
		else r = b;
	}
	return l;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	for(int i = 0; i < 2; i++) {
		cin >> x[i] >> y[i];
	}

	cout << ternarySearch(0, 1000, f) << endl;;
}