#include <bits/stdc++.h>
using namespace std;
double EPS = 1e-11;
double a[4];
double p[4][2] = {{-1, -1}, {1, -1}, {-1, 1}, {1, 1}};
bool poss(double thr) {
  double min03 = numeric_limits<double>::infinity();
  double max03 = -numeric_limits<double>::infinity();
  double min12 = numeric_limits<double>::infinity();
  double max12 = -numeric_limits<double>::infinity();
  for (int i = 0; i < 4; i++) {
    min03 = min(min03, (a[0] + thr * p[i][0]) * (a[3] + thr * p[i][1]));
    min12 = min(min12, (a[1] + thr * p[i][0]) * (a[2] + thr * p[i][1]));
    max03 = max(max03, (a[0] + thr * p[i][0]) * (a[3] + thr * p[i][1]));
    max12 = max(max12, (a[1] + thr * p[i][0]) * (a[2] + thr * p[i][1]));
  }
  return !(max03 < min12 || max12 < min03);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  for (int i = 0; i < 4; i++) cin >> a[i];
  double lo = 0, hi = 1e9;
  for (int i = 0; i < 10000; i++) {
    const double mid = (lo + hi) / 2;
    if (poss(mid))
      hi = mid;
    else
      lo = mid;
  }
  cout << setprecision(20) << lo << endl;
  return 0;
}
