#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <complex>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <iomanip>
#include <assert.h>
#include <array>
#include <cstdio>
#include <cstring>
#include <random>
#include <functional>
#include <numeric>
#include <bitset>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
#define all(c) (c).begin(), (c).end()
#define zero(a) memset(a, 0, sizeof a)
#define minus(a) memset(a, -1, sizeof a)
#define minimize(a, x) a = std::min(a, x)
#define maximize(a, x) a = std::max(a, x)

using ll  = long long;
int const inf = 1<<29;

#define double long double

typedef complex<double> P;

vector<P> v;

double func(double y) {
  P p = P(0, y);
  P ap = v[0] - p;
  P bp = v[1] - p;
  auto aplen = ap.real() * ap.real() + ap.imag() * ap.imag();
  auto bplen = bp.real() * bp.real() + bp.imag() * bp.imag();

//  auto ab = v[0] - v[1];
//  auto ablen = ab.real() * ab.real() + ab.imag() * ab.imag();
  return sqrt(aplen) + sqrt(bplen);// - ablen;
}

int main() {

  rep(i, 2) {
    double x, y; cin >> x >> y;
    v.emplace_back(x, y);
  }

  double l = 0.1, r = 1000.1;
  rep(_, 100) {
    auto ml = (2 * l + r) / 3.0;
    auto mr = (l + 2 * r) / 3.0;

//    cout << ml << " " << mr << endl;

    if(func(ml) <= func(mr)) {
      r = mr;
    }
    else {
      l = ml;
    }
  }

  printf("%.10Lf\n", (l + r) / 2.0);

  return 0;
}