#include <bits/stdc++.h>
using namespace std;
bool compare(int a, int b) { return a > b; }
class Compare {
 public:
  bool operator()(int a, int b) { return a > b; }
};
int mod(int a, int b) { return ((a % b) + b) % b; }
bool intersect(double a, double b, double c, double d) {
  if (a >= c && a <= d) return true;
  if (c >= a && c <= b) return true;
  return false;
}
int main() {
  double a, b, c, d;
  scanf("%lf %lf %lf %lf", &a, &b, &c, &d);
  double lb = 1.0e-19, mid, ub = 1.0e+19;
  for (int i = 0; i < 1.0e+6; i++) {
    mid = ((ub - lb) / 2.0) + lb;
    double mxb1b4 = max((a - mid) * (d - mid),
                        max((a + mid) * (d + mid),
                            max((a - mid) * (d + mid), (a + mid) * (d - mid))));
    double mnb1b4 = min((a - mid) * (d - mid),
                        min((a + mid) * (d + mid),
                            min((a - mid) * (d + mid), (a + mid) * (d - mid))));
    double mxb2b3 = max((b - mid) * (c - mid),
                        max((b + mid) * (c + mid),
                            max((b - mid) * (c + mid), (b + mid) * (c - mid))));
    double mnb2b3 = min((b - mid) * (c - mid),
                        min((b + mid) * (c + mid),
                            min((b - mid) * (c + mid), (b + mid) * (c - mid))));
    if (intersect(mnb1b4, mxb1b4, mnb2b3, mxb2b3))
      ub = mid;
    else
      lb = mid;
  }
  printf("%.9lf", mid);
  return 0;
}
