#include <bits/stdc++.h>
using namespace std;
long long a, b, c, d;
double getLow(double a, double d, double x) {
  double ret = (a - x) * (d - x);
  ret = min(ret, (a - x) * (d + x));
  ret = min(ret, (a + x) * (d - x));
  ret = min(ret, (a + x) * (d + x));
  return ret;
}
double getHigh(double a, double d, double x) {
  double ret = (a - x) * (d - x);
  ret = max(ret, (a - x) * (d + x));
  ret = max(ret, (a + x) * (d - x));
  ret = max(ret, (a + x) * (d + x));
  return ret;
}
bool check(double x) {
  double adLow = getLow(a, d, x);
  double adHigh = getHigh(a, d, x);
  double bcLow = getLow(b, c, x);
  double bcHigh = getHigh(b, c, x);
  if (adLow <= bcHigh && bcHigh <= adHigh) return true;
  if (bcLow <= adHigh && adHigh <= bcHigh) return true;
  return false;
}
int main() {
  scanf("%I64d %I64d %I64d %I64d", &a, &b, &c, &d);
  double lo = 0, hi = max(abs(a - c), abs(d - b));
  for (int it = 0; it < 100; ++it) {
    double mid = (lo + hi) / 2.0;
    if (check(mid))
      hi = mid;
    else
      lo = mid;
  }
  printf("%.10f\n", (lo + hi) / 2.0);
  return 0;
}
