#include <bits/stdc++.h>
using namespace std;
int i;
double a, b, c, d, base, top, mid, x11, x22, y11, y22, minim1, minim2, maxim1,
    maxim2;
int main() {
  scanf("%lf%lf%lf%lf", &a, &b, &c, &d);
  base = 0.0;
  top = 1000000000.0;
  for (i = 1; i <= 100; i++) {
    mid = (base + top) / 2.0;
    x11 = a - mid;
    x22 = a + mid;
    y11 = d - mid;
    y22 = d + mid;
    minim1 = min(min(x11 * y11, x11 * y22), min(x22 * y11, x22 * y22));
    maxim1 = max(max(x11 * y11, x11 * y22), max(x22 * y11, x22 * y22));
    x11 = b - mid;
    x22 = b + mid;
    y11 = c - mid;
    y22 = c + mid;
    minim2 = min(min(x11 * y11, x11 * y22), min(x22 * y11, x22 * y22));
    maxim2 = max(max(x11 * y11, x11 * y22), max(x22 * y11, x22 * y22));
    if (maxim1 < minim2 || maxim2 < minim1)
      base = mid;
    else
      top = mid;
  }
  printf("%.9lf\n", top);
}
