#include <bits/stdc++.h>
using namespace std;
const long long INF = 0x3fffffff;
double a, b, c, d;
int main() {
  while (cin >> a >> b >> c >> d) {
    double l = 0, r = 2000000000.0;
    double a1, a2, b1, b2, c1, c2, d1, d2;
    double m1, m2, min1, min2;
    for (int i = 0; i < 10000; i++) {
      double 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;
      min1 = min(min(a1 * d1, a1 * d2), min(a2 * d1, a2 * d2));
      min2 = min(min(b1 * c1, b1 * c2), min(b2 * c1, b2 * c2));
      m1 = max(max(a1 * d1, a1 * d2), max(a2 * d1, a2 * d2));
      m2 = max(max(b1 * c1, b1 * c2), max(b2 * c1, b2 * c2));
      if (min1 <= m2 && min2 <= m1)
        r = mid;
      else
        l = mid;
    }
    printf("%.10lf\n", l);
  }
  return 0;
}
