### main

while line = gets
  a, l, x = line.split.map(&:to_f)

  h = Math.sqrt(l ** 2 - (a / 2) ** 2)
  abc = a * h / 2

  hx = Math.sqrt(((l + x) / 2) ** 2 - (l / 2) ** 2)
  adc = l * hx / 2

  area = abc + 2 * adc

  puts "%.10f" % area
end