#include <bits/stdc++.h>
using namespace std;
int main() {
  int t = 0;
  double a, b, c, d;
  double l = 0, r = 1000000000000, m;
  double a1, a2, b1, b2, c1, c2, d1, d2, t1, t2, p1, p2;
  scanf("%lf%lf%lf%lf", &a, &b, &c, &d);
  while (t < 1000) {
    m = (l + r) / 2;
    a1 = a - m;
    a2 = a + m;
    d1 = d - m;
    d2 = d + m;
    t1 = min(min(a1 * d1, a2 * d2), min(a1 * d2, a2 * d1));
    t2 = max(max(a1 * d1, a2 * d2), max(a1 * d2, a2 * d1));
    b1 = b - m;
    b2 = b + m;
    c1 = c - m;
    c2 = c + m;
    p1 = min(min(b1 * c1, b2 * c2), min(b1 * c2, b2 * c1));
    p2 = max(max(b1 * c1, b2 * c2), max(b1 * c2, b2 * c1));
    if (t1 < p2 && p1 < t2)
      r = m;
    else
      l = m;
    t++;
  }
  printf("%.10lf\n", l);
  return 0;
}
