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 size
        @parent.map{ |id_x| get_parent(id_x) }.uniq.size
    end

end



n, m = gets.split(" ").map(&:to_i)
c = UnionFind.new(n)
m.times{ |i|
  a, b = gets.split(" ").map(&:to_i)
  c.unite(a-1, b-1)
}
puts c.size - 1