while True:
  try:
    a, l, x = map(int, input().split())
  except EOFError:
    break
  s1 = (l + l + x) / 2
  v1 = (s1 * (l / 2) * (l / 2) * (x / 2)) ** (1 / 2)
  s2 = (l + l + a) / 2
  v2 = (s2 * (s2 - l) * (s2 - l) * (s2 - a)) ** (1 / 2)
  print(v1 * 2 + v2)
