#include <bits/stdc++.h>
using namespace std;

int main(){
	double Xa,Ya,Xb,Yb;
	cin >> Xa >> Ya >> Xb >> Yb;
	double min = INT_MAX;
	double ans;
	for(double i = Ya; i <= Yb; i+=0.000001){
		double d1 = sqrt(Xa * Xa + (i - Ya) * (i - Ya));
		double d2 = sqrt(Xb * Xb + (Yb - i) * (Yb - i));
		//		cout << i << " " << d1+d2 << endl;
		if(min > d1+d2){
			min = d1+d2;
			ans = i;
		}
		else break;
	}
	cout << ans << endl;		
	return 0;
}