import Data.Array (Array, listArray, (!))

flags :: [Int] -> Int
flags a = result
  where
    n   = length a
    arr = listArray (0, n - 1) a :: Array Int Int
    at i
      | i >= 0 && i < n = arr ! i
      | otherwise       = 0

    isPeak i = i > 0 && i < n && arr ! (i - 1) < arr ! i && arr ! i > at (i + 1)

    nextArr :: Array Int Int
    nextArr = listArray (0, n - 1) [compute i | i <- [0 .. n - 1]]
      where
        compute i
          | i == n - 1 = if isPeak i then i else -1
          | isPeak i   = i
          | otherwise  = nextArr ! (i + 1)

    result = go 1 0
    go i best
      | i * (i - 1) > n = best
      | otherwise       = go (i + 1) (max best (countFlags i))

    countFlags i = walk 0 0
      where
        walk pos num
          | not (pos < n && num < i) = num
          | nextArr ! pos == -1      = num
          | otherwise                = walk (nextArr ! pos + i) (num + 1)