class UnionFind

  attr_accessor :nodes, :sizes

  def initialize(num)
    self.nodes = []
    self.sizes = []

    num.times do |n|
      self.nodes[n] = n
      self.sizes[n] = 1
    end
  end

  def root(i)
    # Loop up the chain until reaching root
    while nodes[i] != i do
      # path compression for future lookups
      nodes[i] = nodes[nodes[i]]
      i = nodes[i]
    end
    i
  end

  def union(i, j)
    rooti = root i
    rootj = root j

    # already connected
    return if rooti == rootj

    # root smaller to root of larger
    if sizes[i] < sizes[j]
      nodes[rooti] = rootj
      sizes[rootj] += sizes[rooti]
    else
      nodes[rootj] = rooti
      sizes[rooti] += sizes[rootj]
    end
  end

  def connected?(i, j)
    root(i) == root(j)
  end

end

N, M = gets.split.map &:to_i
ABs = $<.map{|s|
  s.split.map &:to_i
}
uf = UnionFind.new(N+1)

ABs.each{|a, b|
  uf.union(a, b)
}
p (1..N).map{|i|
  uf.root(i)
}.uniq.count - 1