#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#define double long double
using namespace std;

int main()
{
	double x1, y1;
	double x2, y2;
	cin >> x1 >> y1;
	cin >> x2 >> y2;
	
	double pos, mind = 114514;
	for( double y = min(y1, y2); y <= max(y1, y2); y += 1e-4 ){
		double d = hypot(x1, y1 - y) + hypot(x2, y2 - y);
		if( d < mind ){
			mind = d;
			pos = y;
		}
	}
	
	for( double y = pos - 0.01; y <= pos + 0.01; y += 1e-8 ){
		double d = hypot(x1, y1 - y) + hypot(x2, y2 - y);
		if( d < mind ){
			mind = d;
			pos = y;
		}
	}
	printf("%.14Lf\n", pos);
	return 0;
}