#include <bits/stdc++.h>
using namespace std;
const long double INF = 1e9;
long long a, b, c, d;
long long sgn(long long x) {
  if (x == 0) return 0;
  if (x > 0) return 1;
  return -1;
}
int main() {
  cout << fixed << setprecision(10);
  cin >> a >> b >> c >> d;
  long long det = a * d - b * c;
  if (det < 0) {
    det *= -1;
    swap(a, b);
    swap(c, d);
  }
  long double L = 0.0L, R = INF, M;
  for (int i = int(0); i < int(100); ++i) {
    M = (L + R) / 2;
    bool good = false;
    for (int aa = -1; aa < 2; aa += 2)
      for (int bb = -1; bb < 2; bb += 2)
        for (int cc = -1; cc < 2; cc += 2)
          for (int dd = -1; dd < 2; dd += 2)
            if ((a + M * aa) * (d + M * dd) - (b + M * bb) * (c + M * cc) < 0) {
              good = true;
            }
    if (good)
      R = M;
    else
      L = M;
  }
  cout << (double)L << '\n';
  return 0;
}
