#include<iostream>
#include<cstdio>
#include<cmath>

using namespace std;

double xa, ya, xb, yb;
double f(double py){
	return sqrt(pow(xa - 0, 2) + pow(ya - py, 2)) + sqrt(pow(xb - 0, 2) + pow(yb - py, 2));
}
double search(double l, double r,int cnt){
	while(cnt--){
		if(f((l*2+r)/3)<f((l+r*2)/3)) r=(l+r*2)/3;
		else l=(l*2+r)/3;
	}
	return (l+r)*0.5;
}


int main(){


	cin >> xa >> ya;
	cin >> xb >> yb;

	printf("%.12f\n",search(0, 1000, 10000));
}