#include <bits/stdc++.h>

using namespace std;

#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()

template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1ll<<29;
const ll MOD = 1000000007;
const double EPS  = 1e-10;

int main() {
	int xa, ya, xb, yb;
	cin >> xa >> ya >> xb >> yb;
	
	long double l = 0, r = 2000;
	
	REP(i, 100) {
		long double c1 = l + (r - l) / 3;
		long double c2 = r - (r - l) / 3;
		
		long double y1 = hypot<long double>(xa, ya - c1) + hypot<long double>(xb, yb - c1);
		long double y2 = hypot<long double>(xa, ya - c2) + hypot<long double>(xb, yb - c2);
		
		if (y1 < y2) r = c2;
		else l = c1;
	}
	
	printf("%.15f\n", (double)r);
	return 0;
}