import java.io.*;
import java.util.*;

public class Main{
    public static void main(String[] args) throws Exception {
        BufferedReader in=new BufferedReader(new FileReader("input.txt"));
        FileWriter writer=new FileWriter("output.txt");
        String s=in.readLine();
        int n=Integer.parseInt(s);
        s=in.readLine();
        int m=Integer.parseInt(s);
        ArrayList<LinkedList<Integer>> lists=new ArrayList<>(n);
        for(int i=0;i<n;i++){
        	lists.add(new LinkedList<>());
        }
        boolean[] visit=new boolean[n];
        int ans=0;
        for(int i=0;i<m;i++){
            s=in.readLine();
            StringTokenizer st=new StringTokenizer(s," ");
            int a=Integer.parseInt(st.nextToken())-1;
            int b=Integer.parseInt(st.nextToken())-1;
            lists.get(a).add(b);
            lists.get(b).add(a);
        }
        Stack<Integer> stack=new Stack<>();
        for(int i=0;i<n;i++){
            if(!visit[i]){
            	visit[i]=true;
            	ans++;
                stack.add(i);
                while(!stack.isEmpty()){
                    int el=stack.pop();
                    for(Integer j:lists.get(el)){
                        if(!visit[j]){
                        	stack.add(j);
                            visit[j]=true;
                        }
                    }
                }
            }
        }
        writer.write(Integer.toString(--ans));
        writer.flush();
    }
}