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

int main() {
  int xa, ya, xb, yb;
  cin >> xa >> ya >> xb >> yb;
  long double l = 0, r = 1000;
  auto f = [&](long double mid) {
    return hypot(xa, ya - mid) + hypot(xb, yb - mid);
  };
  for (int i = 300; i; i--) {
    long double mid1 = (l * 2 + r) / 3;
    long double mid2 = (l + r * 2) / 3;
    (f(mid1) < f(mid2)) ? r = mid2 : l = mid1;
  }
  cout << fixed << setprecision(6) << l << "\n";
}