#include <bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
using namespace std;

int main(){
	double a,b,c,d;cin>>a>>b>>c>>d;
	double l=0,r=1000;
	rep(i,100){
		double a1=l+(r-l)/3;
		double b1=l+(r-l)/3*2;
		if((b-a1)*(b-a1)+(d-a1)*(d-a1)<(b-b1)*(b-b1)+(d-b1)*(d-b1))r=b1;
		else l=a1;
	}
	printf("%.12lf\n",l);
}