#include <bits/stdc++.h>
using namespace std;
int a, b, c, d;
double getMax(double x, double y, double d) {
  double temp = (x - d) * (y + d);
  temp = max((x - d) * (y - d), temp);
  temp = max((x + d) * (y - d), temp);
  temp = max((x + d) * (y + d), temp);
  return temp;
}
double getMin(double x, double y, double d) {
  double temp = (x - d) * (y - d);
  temp = min((x - d) * (y + d), temp);
  temp = min((x + d) * (y - d), temp);
  temp = min((x + d) * (y + d), temp);
  return temp;
}
bool check(double x) {
  double l1 = getMin(a, d, x);
  double r1 = getMax(a, d, x);
  double l2 = getMin(b, c, x);
  double r2 = getMax(b, c, x);
  if (l1 <= r2 && r1 >= l2) return true;
  return false;
}
int main() {
  while (~scanf("%d%d%d%d", &a, &b, &c, &d)) {
    double l, r, mid;
    r = max(max(abs(a), abs(b)), max(abs(c), abs(d)));
    l = 0;
    int num = 1000000;
    while (num--) {
      mid = (l + r) / 2.0;
      if (check(mid))
        r = mid;
      else
        l = mid;
    }
    printf("%.10lf\n", l);
  }
}
