#include <bits/stdc++.h>
using namespace std;
long double A[4];
long double find_det() { return A[0] * A[3] - A[1] * A[2]; }
int main(void) {
  for (int i = 0; i < 4; i++) {
    cin >> A[i];
  }
  long double maxim = max(
      max(fabs(A[0] + A[1] + A[2] + A[3]), fabs(A[0] - A[1] + A[2] - A[3])),
      max(fabs(A[0] - A[1] - A[2] + A[3]), fabs(A[0] + A[1] - A[2] - A[3])));
  if (maxim == 0)
    cout << setprecision(12) << 0 << endl;
  else
    cout << setprecision(12) << fabs(find_det()) / fabs(maxim) << endl;
  ;
  return 0;
}
