let ``No.29 パワーアップ``() =

  let sum = Array.zeroCreate<int> 11
  [1.. (stdin.ReadLine() |> int) ]
  |> List.iter (fun i ->

    stdin.ReadLine().Split(' ')
    |> Array.map int
    |> Array.iter (fun x -> sum.[x] <- sum.[x] + 1)
  )

  (sum |> Array.sumBy (fun e -> e/2 )) + (sum |> Array.sumBy (fun e -> e%2 )) / 4
  |> stdout.WriteLine


``No.29 パワーアップ``()