Problem:
Score : 300 points

Problem Statement

There are N cities numbered 1 through N, and M bidirectional roads numbered 1 through M.
Road i connects City A_i and City B_i.

Snuke can perform the following operation zero or more times:

Choose two distinct cities that are not directly connected by a road, and build a new road between the two cities.

After he finishes the operations, it must be possible to travel from any city to any other cities by following roads (possibly multiple times).

What is the minimum number of roads he must build to achieve the goal?

Constraints

2 \leq N \leq 100,000

1 \leq M \leq 100,000

1 \leq A_i < B_i \leq N

No two roads connect the same pair of cities.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
:
A_M B_M

Output

Print the answer.

Sample Input 1

3 1
1 2

Sample Output 1

1

Initially, there are three cities, and there is a road between City 1 and City 2.

Snuke can achieve the goal by building one new road, for example, between City 1 and City 3.
After that,

We can travel between 1 and 2 directly.

We can travel between 1 and 3 directly.

We can travel between 2 and 3 by following both roads (2 - 1 - 3).

Solution in Java (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.*;
 
public class Main{
    static class FastReader{
        BufferedReader br;
        StringTokenizer st;
        public FastReader(){br = new BufferedReader(new InputStreamReader(System.in));}
        public FastReader(String fileName) throws FileNotFoundException{br = new BufferedReader(new FileReader(fileName));}
        String next(){while (st == null || !st.hasMoreElements()){try{st = new StringTokenizer(br.readLine());}catch (IOException  e){e.printStackTrace();}}return st.nextToken();}
        int nextInt(){return Integer.parseInt(next());}
        long nextLong(){return Long.parseLong(next());}
        double nextDouble(){return Double.parseDouble(next());}
        char[][] readCharArray(int n,int m) {
            String str;
            char[][] arr = new char[n][m];
            for(int i=0;i<n;i++) {
                str=nextLine();
                for(int j=0;j<n;j++) {
                    arr[i][j]=str.charAt(j);
                }
            }
            return arr;
        }
        char[][] readCharArray(int n){
            return readCharArray(n,n);
        }
        int[] readArray(int n){int[] a=new int[n];for (int i=0; i<n; i++) a[i]=nextInt();return a;}
        long[] readLongArray(int n){long[] a=new long[n];for (int i=0; i<n; i++) a[i]=nextLong();return a;}
        String nextLine(){String str = "";try{str = br.readLine();}catch (IOException e){e.printStackTrace();}return str;}
    }
    static void printArray(int[] arr){
        for(int x:arr){
            System.out.print(x+" ");
        }
        System.out.println();
    }
    static final Random random=new Random();
    static void ruffleSort(int[] a) {
        int n=a.length;//shuffle, then sort
        for (int i=0; i<n; i++) {
            int oi=random.nextInt(n), temp=a[oi];
            a[oi]=a[i]; a[i]=temp;
        }
        Arrays.sort(a);
    }
    public static void main(String[] args) throws NumberFormatException, IOException {
        FastReader st = new FastReader();
        int ans=0;
        int n = st.nextInt();
        int m = st.nextInt();
        Node[] node = new Node[n];
        for(int i=0;i<n;i++) node[i] = new Node();
        for(int i=0;i<m;i++) {
        	int a = st.nextInt(),b = st.nextInt();
        	node[a-1].adj.add(node[b-1]);
        	node[b-1].adj.add(node[a-1]);
        }
        dfs(node[0]);
        for(int i=0;i<n;i++) {
        	if(!node[i].vis) {
        		dfs(node[i]);
        		ans++;
        	}
        }
        System.out.println(ans);
    }
    static void dfs(Node from) {
    	from.vis = true;
    	for(Node nn:from.adj) {
    		if(!nn.vis) {
    			nn.vis = true;
    			dfs(nn);
    		}
    	}
    }
    static class Node{
    	boolean vis = false;
    	ArrayList<Node> adj = new ArrayList<>();
    }
}