Monkey and river (optimal)

def solution(a, d)
  return 0 if d > a.length

  position = -1
  time = 0
  max_time = 0
  length = a.size

  while true do
    start_position = position + 1
    end_position = position + d
    new_position = -2

    while new_position != position
      return time if end_position >= length

      new_position = position
      unreach_count = 0

      end_position.downto(start_position) do |index|
        max_time = [max_time, a[index]].max

        if a[index] == -1
          unreach_count += 1

          return -1 if unreach_count >= d
        end

        if a[index] <= time && a[index] != -1
          position = index
          break
        end
      end

      start_position = position + 1
      end_position = [position + d, length - 1].min
    end

    if position + d + 1 > length
      return [max_time, time].min
    end

    time += 1

    return [max_time, time].min if time > length
  end
end