#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double yd, yu, yw, xr, yr, R, pi = acos(-1), t = 0;
  cin >> yd >> yu >> yw >> xr >> yr >> R;
  double l = 0, r = xr, mid, dist;
  while (r - l > 1e-8) {
    mid = l + (r - l) / 2.0;
    double deg = acos((yw - yr - R) / sqrt((xr - mid) * (xr - mid) +
                                           (yw - yr - R) * (yw - yr - R)));
    deg = pi / 2 - deg;
    double tn = tan(deg);
    double y = tn * (2 * (xr - mid) - xr) + yr;
    dist = abs(-yu + tn * (2 * (xr - mid) - xr) + yr) / sqrt(tn * tn + 1);
    if (y < yd + R) {
      r = mid;
      continue;
    } else
      l = mid;
  }
  if (dist >= R)
    cout << setprecision(20) << l << endl;
  else
    cout << -1;
}
