#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-10;
const double INF = 1e10;
double a, b, c, d;
bool check(double x) {
  double ad_low = min(min((a - x) * (d - x), (a - x) * (d + x)),
                      min((a + x) * (d - x), (a + x) * (d + x)));
  double ad_high = max(max((a - x) * (d - x), (a - x) * (d + x)),
                       max((a + x) * (d - x), (a + x) * (d + x)));
  double bc_low = min(min((b - x) * (c - x), (b - x) * (c + x)),
                      min((b + x) * (c - x), (b + x) * (c + x)));
  double bc_high = max(max((b - x) * (c - x), (b - x) * (c + x)),
                       max((b + x) * (c - x), (b + x) * (c + x)));
  if (ad_high < bc_low || bc_high < ad_low)
    return false;
  else
    return true;
}
int main() {
  cin >> a >> b >> c >> d;
  double L = 0, R = INF;
  double M;
  int cnt = 0;
  cout << fixed << setprecision(10);
  while (cnt < 100) {
    cnt++;
    M = (L + R) / 2;
    if (check(M))
      R = M;
    else
      L = M;
  }
  cout << M << endl;
}
