lib C
  fun strtoll(s : UInt8*, p : UInt8**, b : Int32) : Int64
end

class String
  def to_i64
    C.strtoll(self, nil, 10)
  end
end

n = read_line.to_i
a = (1..n).map { read_line.to_i }.sort

q = Deque.new(a[...1])

a[1..].each do |i|
  if q[0] + 1 < i
    q.shift
  end
  q.push(i)
end

puts q.size