#include <bits/stdc++.h>
double a, b, c, d;
double min(double a, double b) {
  if (a > b)
    return b;
  else
    return a;
}
double max(double a, double b) {
  if (a > b)
    return a;
  else
    return b;
}
int main() {
  int i, j, n, k;
  double t1, t2, x1, x2;
  double a1, a2, b1, b2, c1, c2, d1, d2;
  while (scanf("%lf%lf%lf%lf", &a, &b, &c, &d) != EOF) {
    double t;
    double l = 0, r = 1000000000, mid;
    for (i = 0; i < 100000; i++) {
      mid = (l + r) / 2;
      a1 = a + mid;
      a2 = a - mid;
      b1 = b + mid;
      b2 = b - mid;
      c1 = c + mid;
      c2 = c - mid;
      d1 = d + mid;
      d2 = d - mid;
      t1 = min(min(a1 * d1, a1 * d2), min(a2 * d1, a2 * d2));
      t2 = min(min(b1 * c1, b1 * c2), min(b2 * c1, b2 * c2));
      x1 = max(max(a1 * d1, a1 * d2), max(a2 * d1, a2 * d2));
      x2 = max(max(b1 * c1, b1 * c2), max(b2 * c1, b2 * c2));
      if (t1 <= x2 && t2 <= x1)
        r = mid;
      else
        l = mid;
    }
    printf("%.11f\n", l);
  }
  return 0;
}
