#include <iostream>
#include <cstdio>
#include <cmath>
#include <cassert>
using namespace std;
int main(){
	int X_a,X_b;
	int Y_a,Y_b;
	cin >> X_a >> Y_a;
	cin >> X_b >> Y_b;

	assert(0 < X_a && X_a < 1000);
	assert(0 < Y_a && Y_a < 1000);
	assert(0 < X_b && X_b < 1000);
	assert(0 < Y_b && Y_b < 1000);

	auto func = [&](long double y){
		long double ret = 0;
		ret += sqrt(X_a*X_a + (Y_a-y)*(Y_a-y));
		ret += sqrt(X_b*X_b + (Y_b-y)*(Y_b-y));
		return ret;
	};

	long double lb = min(Y_a, Y_b);
	long double ub = max(Y_a, Y_b);
	while(ub-lb > 1e-9){
		long double l = (2*lb + ub)/3.0;
		long double r = (lb + 2*ub)/3.0;
		if(func(l)>func(r)){
			lb = l;
		}else{
			ub = r;
		}
	}

	printf("%.12f\n", (double)lb);
	return 0;
}