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 11.0.6) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.StringTokenizer;

public class Main {

	public static void main(String[] args) throws IOException {
		Main mainObj = new Main();
		mainObj.solve();
	}

	public void solve() throws IOException {
		FastScanner fs = new FastScanner();
		
		int n = fs.nextInt();
		int m = fs.nextInt();
		
		City[] cityArr = new City[n];
		int[] groupArr = new int[n];
		
		for(int i = 0; i < n; i++) {
			cityArr[i] = new City(i);
		}
		
		for(int i = 0; i < m; i++) {
			int a = fs.nextInt() - 1;
			int b = fs.nextInt() - 1;
			
			cityArr[a].addLoad(b);
			cityArr[b].addLoad(a);
		}
		
		int groupNum = 0;
		for(int i = 0; i < n; i++) {
			if (groupArr[i] != 0) {
				continue;
			}
			
			if(cityArr[i].load.isEmpty()) {
				continue;
			}
			
			groupNum++;
			
			groupArr[i] = groupNum;
			
			ArrayDeque<Integer> queue = new ArrayDeque<>();
			queue.addAll(cityArr[i].load);
			while(!queue.isEmpty()) {
				int cityId = queue.poll();
				if(groupArr[cityId] != 0) {
					continue;
				}
				
				groupArr[cityId] = groupNum;
				queue.addAll(cityArr[cityId].load);
			}
		}
		for(int i = 0; i < n; i++) {
			if(groupArr[i] == 0) {
				groupNum++;
			}
		}
		System.out.println(groupNum-1);
	}
	
	class City {
		int id;
		HashSet<Integer> load;
		
		City(int id){
			this.id = id;
			this.load = new HashSet<>();
		}
		
		void addLoad(int id) {
			load.add(id);
		}
	}

	

	public class FastScanner {

		BufferedReader reader;
		private StringTokenizer st;

		public FastScanner() {
			st = null;
			reader = new BufferedReader(new InputStreamReader(System.in));
		}

		public String next() throws IOException {
			if (st == null || !st.hasMoreElements()) {
				st = new StringTokenizer(reader.readLine());
			}
			return st.nextToken();
		}

		public String nextLine() throws IOException {
			st = null;
			String readLine = null;
			readLine = reader.readLine();
			return readLine;
		}

		public int nextInt() throws NumberFormatException, IOException {
			return Integer.parseInt(next());
		}

		public long nextLong() throws NumberFormatException, IOException {
			return Long.parseLong(next());
		}

		public int[] nextIntArr(int n) throws NumberFormatException, IOException {
			int[] retArr = new int[n];
			for (int i = 0; i < n; i++) {
				retArr[i] = nextInt();
			}
			return retArr;
		}

		public long[] nextLongArr(int n) throws NumberFormatException, IOException {
			long[] retArr = new long[n];
			for (int i = 0; i < n; i++) {
				retArr[i] = nextLong();
			}
			return retArr;
		}

		public void close() throws IOException {
			reader.close();
		}
	}

}