#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, b, c, d;
  cin >> a >> b >> c >> d;
  double result = max(0.0, fabs(a + b + c + d));
  result = max(result, fabs(a - b - c + d));
  result = max(result, fabs(a - b + c - d));
  result = max(result, fabs(a + b - c - d));
  result = result ? fabs(a * d - b * c) / result : result;
  cout << setprecision(9) << fixed << result;
  return 0;
}
