class Node
  attr_accessor :parent, :rank

  def initialize(n)
    @parent = n
    @rank = 0
  end
end

class UnionFindTree
  def initialize(n)
    @nodes = (0..n).to_a.map { |i| Node.new(i) }
  end

  def find(x)
    return x if @nodes[x].parent == x

    return @nodes[x].parent = find(@nodes[x].parent)
  end

  def unite(a, b)
    a = find(a)
    b = find(b)
    return if a == b

    if @nodes[a].rank < @nodes[b].rank
      @nodes[a].parent = b
    else
      @nodes[b].parent = a
      @nodes[a].rank += 1 if @nodes[a].rank == @nodes[b].rank
    end
  end

  def same?(a, b)
    find(a) == find(b)
  end

  def parents
    @nodes.map(&:parent)
  end
end
N,M=gets.split.map &:to_i
as=M.times.map{gets.split.map(&:to_i)}
tree=UnionFindTree.new(N)
for i in 0...M
    tree.unite(as[i][0],as[i][1])
end
puts tree.parents[1..-1].uniq.size
