11724.java

import java.util.*;

public class Main {

 public static void main(String[] args) {

 Scanner sc = new Scanner(System.in);

 int n = sc.nextInt();
 int m = sc.nextInt();

 List<List<Integer>> nodes = new ArrayList<>(n);
 for (int i = 0; i < n + 1; i++) {
 nodes.add(new ArrayList<>());
 }

 for (int i = 0; i < m; i++) {
 int a = sc.nextInt();
 int b = sc.nextInt();
 nodes.get(a).add(b);
 nodes.get(b).add(a);
 }

 int cnt = 0;
 List<Integer> visited = new ArrayList<>();

 while(visited.size() != n) {
 cnt++;

 for(int i=1; i<=n; i++) {
 if(visited.contains(i))
 continue;
 visited = bfs(nodes, n, i, visited);
 break;
 }
 }

 System.out.println(cnt);
 }


 public static List<Integer> bfs(List nodes, int n, int start, List<Integer> visited) {

 Queue<Integer> q = new LinkedList<>();
 boolean[] check = new boolean[n+1];
 q.offer(start);

 while (!(q.isEmpty())) {
 int now = q.poll();

 if (check[now])
 continue;

 visited.add(now);
 check[now] = true;

 for (Object num : (List) nodes.get(now)) {
 int next = (int) num;
 q.offer(next);
 }
 }

 return visited;
 }
}