N = gets.to_i
A = gets.split.map(&:to_i)
C = A.tally
C.default = 0

ans = 0

0.upto(A.max) do |a|
  has_three = true if C[a] >= 3

  if C[a] <= 1
    if C[a] == 1
      if has_three
        ans += 1
      elsif (a + 1..A.max).any? { |x| C[x] >= 1 }
        ans += 1
      end
    end

    break
  end

  ans += 1
end

puts ans