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

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> PP;

//-----------------------------------------------------------------------------

P p1,p2;

double f(double mid) {
	double dist=sqrt(p1.fi*p1.fi+abs(p1.se-mid)*abs(p1.se-mid));
	dist+=sqrt(p2.fi*p2.fi+abs(p2.se-mid)*abs(p2.se-mid));
	return dist;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>p1.fi>>p1.se>>p2.fi>>p2.se;
	double lb=0,ub=1010;
	REP(i,100) {
		if(f((lb*2+ub)/3)<f((lb+ub*2)/3)) ub=(lb+ub*2)/3;
		else lb=(lb*2+ub)/3;
	}
	printf("%.10f\n",(lb+ub)/2);

	return 0;
}