#include <iostream>
#include <string>
#include <cmath>
using namespace std;

double min(double a, double b)
{
	if (a > b) return b;
	else return a;
}
double max(double a, double b)
{
	if (a < b) return b;
	else return a;
}

int main()
{
	double ax, ay, bx, by;
	cin >> ax >> ay;
	cin >> bx >> by;

	if (ay == by) {
		cout << ay << endl;
		return 0;
	}
	else if (ax == bx) {
		cout << (ay + by) / 2 << endl;
		return 0;
	}
	else {
		double tmp = 1000000.0;
		double p = min(ay, by);
		
		for (double d = min(ay,by); d <= max(ay,by); d = d + 0.00001) {
			double dis = sqrt(ax*ax + (ay - d)*(ay - d)) + sqrt(bx*bx + (by - d)*(by - d));
			if (dis < tmp) {
				p = d;
				tmp = dis;
			}
		}

		tmp = 1000000.0;
		for (double d = p - 0.00005; d <= p + 0.00005; d = d + 0.000001) {
			double dis = sqrt(ax*ax + (ay - d)*(ay - d)) + sqrt(bx*bx + (by - d)*(by - d));
			if (dis < tmp) {
				p = d;
				tmp = dis;
			}
		}

		cout << p << endl;
	}

	return 0;
}