#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d, maxad, minad, maxbc, minbc, ad1, ad2, ad3, ad4, bc1, bc2,
      bc3, bc4, mid;
  cin >> a >> b >> c >> d;
  double r = 1e9, l = 0, times = 1000;
  while (times--) {
    mid = r + (l - r) / 2;
    ad1 = (a + mid) * (d + mid);
    ad2 = (a + mid) * (d - mid);
    ad3 = (a - mid) * (d + mid);
    ad4 = (a - mid) * (d - mid);
    bc1 = (b + mid) * (c + mid);
    bc2 = (b + mid) * (c - mid);
    bc3 = (b - mid) * (c + mid);
    bc4 = (b - mid) * (c - mid);
    maxad = max(max(ad1, ad2), max(ad3, ad4));
    minad = min(min(ad1, ad2), min(ad3, ad4));
    maxbc = max(max(bc1, bc2), max(bc3, bc4));
    minbc = min(min(bc1, bc2), min(bc3, bc4));
    if (maxad - minbc >= 0 && minad - maxbc <= 0)
      r = mid;
    else
      l = mid;
  }
  printf("%.10lf", l);
  return 0;
}
