#include <bits/stdc++.h>
using namespace std;
const long long N = 1e2 + 9;
long double a, b, c, d;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cerr << "‌";
  cin >> a >> b >> d >> c;
  long double lo = 0, hi = 1e9;
  for (int i = 0; i < (100000); i++) {
    long double mid = (hi + lo) / 2;
    vector<pair<long double, long long> > op;
    op.push_back({(a + mid) * (c + mid), 0});
    op.push_back({(a + mid) * (c - mid), 0});
    op.push_back({(a - mid) * (c + mid), 0});
    op.push_back({(a - mid) * (c - mid), 0});
    op.push_back({(b + mid) * (d + mid), 1});
    op.push_back({(b + mid) * (d - mid), 1});
    op.push_back({(b - mid) * (d + mid), 1});
    op.push_back({(b - mid) * (d - mid), 1});
    sort(op.begin(), op.end());
    long long ok = 0;
    for (int j = 0; j < (4); j++) ok += op[j].second;
    if (ok != 0 && ok != 4)
      hi = mid;
    else
      lo = mid;
  }
  cout << fixed << setprecision(12) << lo << '\n';
}
