let ``No.89 どんどんドーナツどーんといこう！``() =

  let c = stdin.ReadLine() |> float
  let ri , ro =
    stdin.ReadLine().Split(' ')
    |> Array.map (float)
    |> fun args -> args.[0], args.[1]

  let cirArea  rout rin   = 0.25 * System.Math.PI ** 2.0 * (rout + rin) * (rout - rin) * (rout - rin)
  (cirArea ro ri) * c
  |> stdout.WriteLine
  

  ()
``No.89 どんどんドーナツどーんといこう！``()