# 整数の入力
n, m = gets.chomp.split(" ").map(&:to_i)


class UnionFind
  def initialize(size)
    @rank = Array.new(size, 0)
    @parent = Array.new(size, &:itself)
  end 

  def unite(id_x, id_y)
    x_parent = get_parent(id_x)
    y_parent = get_parent(id_y)
    return if x_parent == y_parent

    if @rank[x_parent] > @rank[y_parent]
      @parent[y_parent] = x_parent
    else
      @parent[x_parent] = y_parent
      @rank[y_parent] += 1 if @rank[x_parent] == @rank[y_parent]
    end 
  end 

  def get_parent(id_x)
    @parent[id_x] == id_x ? id_x : (@parent[id_x] = get_parent(@parent[id_x]))
  end 

  def same_parent?(id_x, id_y)
    get_parent(id_x) == get_parent(id_y)
  end 

  def count_parent
    rtn = 0 
    @parent.each_with_index do |p, index|
      rtn += 1 if p == index
    end 
    rtn 
  end 
end

union = UnionFind.new n

# クエリの回数だけ、入力受け取りを回す
m.times do
  a, b = gets.split.map{|x| x.to_i - 1}
  union.unite(a, b)
end

puts union.count_parent - 1 