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: Wrong Answer)

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Deque;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Random;
import java.util.Scanner;

class BIT {
	long bit[] = new long[0];
	int N = 0;

	BIT(int n) {
		bit = new long[n + 1];
		N = n;
	}

	void add(int a, long w) {
		for (int x = a; x < N; x += (x & -x))
			bit[x] += w;
	}

	long sum(int a) {
		long ret = 0;
		for (int x = a; x > 0; x -= x & -x)
			ret += bit[x];
		return ret;
	}

	long modsum(int a, int mod) {
		long ret = 0;
		for (int x = a; x > 0; x -= x & -x) {
			ret += bit[x];
			ret %= mod;
		}
		return ret;
	}
}
class MODBIT {
	long bit[] = new long[0];
	int N = 0;
	int mod=0;
	MODBIT(int n,int m) {
		bit = new long[n + 1];
		N = n;
		mod=m;
	}

	void add(int a, long w) {
		for (int x = a; x < N; x += (x & -x)) {
			bit[x] += w;
			bit[x] %= mod;
		}
	}

	long modsum(int a, long mod2) {
		long ret = 0;
		for (int x = a; x > 0; x -= x & -x) {
			ret += bit[x];
			ret %= mod2;
		}
		return ret;
	}
}

class FastScanner {
	private final InputStream in = System.in;
	private final byte[] buffer = new byte[1024];
	private int ptr = 0;
	private int buflen = 0;

	private boolean hasNextByte() {
		if (ptr < buflen) {
			return true;
		} else {
			ptr = 0;
			try {
				buflen = in.read(buffer);
			} catch (IOException e) {
				e.printStackTrace();
			}
			if (buflen <= 0) {
				return false;
			}
		}
		return true;
	}

	private int readByte() {
		if (hasNextByte())
			return buffer[ptr++];
		else
			return -1;
	}

	private static boolean isPrintableChar(int c) {
		return 33 <= c && c <= 126;
	}

	public boolean hasNext() {
		while (hasNextByte() && !isPrintableChar(buffer[ptr]))
			ptr++;
		return hasNextByte();
	}

	public String next() {
		if (!hasNext())
			throw new NoSuchElementException();
		StringBuilder sb = new StringBuilder();
		int b = readByte();
		while (isPrintableChar(b)) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	public long nextLong() {
		if (!hasNext())
			throw new NoSuchElementException();
		long n = 0;
		boolean minus = false;
		int b = readByte();
		if (b == '-') {
			minus = true;
			b = readByte();
		}
		if (b < '0' || '9' < b) {
			throw new NumberFormatException();
		}
		while (true) {
			if ('0' <= b && b <= '9') {
				n *= 10;
				n += b - '0';
			} else if (b == -1 || !isPrintableChar(b)) {
				return minus ? -n : n;
			} else {
				throw new NumberFormatException();
			}
			b = readByte();
		}
	}

	public int nextInt() {
		long nl = nextLong();
		if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
			throw new NumberFormatException();
		return (int) nl;
	}

	public double nextDouble() {
		return Double.parseDouble(next());
	}

	public char nextchar() {
		return next().charAt(0);
	}
}

class UnionFind {
	int Parent[];

	UnionFind(int n) {// Initialize by -1
		Parent = new int[n];
		Arrays.fill(Parent, -1);
	}

	int root(int A) {// In which tree is A?
		if (Parent[A] < 0)
			return A;
		return Parent[A] = root(Parent[A]);
	}

	int size(int A) {// size of tree which is include A
		return -Parent[root(A)];
	}

	boolean connect(int A, int B) {// Connect A and B
		A = root(A);
		B = root(B);
		if (A == B)
			return false;
		if (size(A) < size(B)) {
			int C = 0;
			C = B;
			B = A;
			A = C;
		} // SWAP
		Parent[A] += Parent[B];
		Parent[B] = A;
		return true;
	}
}

class Pair<T, E> {
	public T first;
	public E second;

	void set(T x, E y) {
		first = x;
		second = y;
	}
}

class Pint {
	@Override
	public boolean equals(Object obj) {
		if (obj instanceof Pint) {
			Pint other = (Pint) obj;
			return other.first == this.first && other.second == this.second;
		}
		return false;
	}

	@Override
	public int hashCode() {
		return Objects.hash(this.first, this.second);
	}

	public int first;
	public int second;

	Pint(int x, int y) {
		first = x;
		second = y;
	}

	void set(int x, int y) {
		first = x;
		second = y;
	}
}

class Tpair {
	public int first;
	public int second;
	public long third;

	Tpair(int x, int y, long z) {
		first = x;
		second = y;
		third = z;
	}

	void set(int x, int y, long z) {
		first = x;
		second = y;
		third = z;
	}
}
class Comb{
	int N=0;
	int mod=0;
	long fact[],ifact[],inv[];
	Comb(int n,int m){
		N=n;
		mod=m;
		fact=new long[N+10];
		ifact=new long[N+10];
		inv=new long[N+10];
		inv[1] = 1;
		for (int i = 2; i < N; i++) inv[i] = 1l * (mod - mod / i) * inv[mod % i] % mod;
		fact[0] = ifact[0] = 1;
		for (int i = 1; i < N; i++) {
			fact[i] = 1l * fact[i - 1] * i % mod;
			ifact[i] = 1l * ifact[i - 1] * inv[i] % mod;
		}
	}
	public long C(int x,int y) {
		if (x < 0 || y < 0 || x < y) return 0;
		return 1l * fact[x] * ifact[y] % mod * ifact[x - y] % mod;
	}
}
public class Main {
	static FastScanner scan = new FastScanner();
	static Scanner scanner = new Scanner(System.in);
	static Random rand = new Random();
	static int mod = 1000000007;
	static double eps = 1.0E-14;
	static int big = Integer.MAX_VALUE;
	static double PI = 3.141592653589793;

	static long modlcm(long a, long b) {
		return a * b * modinv(GCD(a, b), mod);
	}

	static long GCD(long a, long b) {
		return b > 0 ? GCD(b, a % b) : a;
	}

	static long lcm(long a, long b) {
		return a * b / GCD(a, b);
	}

	static int min(int a, int b) {
		return a < b ? a : b;
	}

	static long factorial(int i) {
		return i == 1 ? 1 : i * factorial(i - 1);
	}

	static int max(int... i) {
		int x = i[0];
		for (int e : i)
			x = Math.max(x, e);
		return x;
	}

	static int min(int... i) {
		int x = i[0];
		for (int e : i)
			x = Math.min(x, e);
		return x;
	}

	static long gcd(long... i) {
		long x = i[0];
		for (long e : i)
			x = GCD(x, e);
		return x;
	}

	static long lmax(long... i) {
		long x = i[0];
		for (long e : i)
			x = Math.max(x, e);
		return x;
	}

	static long lmin(long... i) {
		long x = i[0];
		for (long e : i)
			x = Math.min(x, e);
		return x;
	}

	static long nCr(long n, long r, long m) {
		long ans = 1;
		for (long i = 0; i < r; i++) {
			ans *= (n - i);
			ans %= m;
		}
		for (long i = 0; i <= r; i++) {
			ans *= modinv(i, m);
			ans %= mod;
		}
		return ans;
	}

	static int lower_bound(long[] b, long cost) {
		int ok = b.length;
		int ng = -1;
		while (Math.abs(ok - ng) > 1) {
			int mid = (ok + ng) / 2;
			if (b[mid] >= cost)
				ok = mid;
			else
				ng = mid;
		}
		return ok;
	}

	static int upper_bound(int[] b, int cost) {
		int ok = b.length;
		int ng = -1;
		while (Math.abs(ok - ng) > 1) {
			int mid = (ok + ng) / 2;
			if (b[mid] > cost)
				ok = mid;
			else
				ng = mid;
		}
		return ok;
	}

	static boolean isPrime(long n) {
		if (n == 2)
			return true;
		if (n < 2 || n % 2 == 0)
			return false;
		double d = Math.sqrt(n);
		for (int i = 3; i <= d; i += 2)
			if (n % i == 0) {
				return false;
			}
		return true;
	}

	static int upper_division(int a, int b) {
		if (a % b == 0) {
			return a / b;
		} else {
			return a / b + 1;
		}
	}

	static long lupper_division(long a, long b) {
		if (a % b == 0) {
			return a / b;
		} else {
			return a / b + 1;
		}
	}

	static int[] setArray(int a) {
		int b[] = new int[a];
		for (int i = 0; i < a; i++) {
			b[i] = scan.nextInt();
		}
		return b;
	}

	static long[] lsetArray(int a) {
		long b[] = new long[a];
		for (int i = 0; i < a; i++) {
			b[i] = scan.nextLong();
		}
		return b;
	}

	static String reverse(String str) {
		char ch[] = new char[str.length()];
		char chch[] = str.toCharArray();
		int a = str.length();
		for (int i = 0; i < upper_division(a, 2); i++) {
			ch[i] = chch[ch.length - i - 1];
			ch[ch.length - 1 - i] = chch[i];
		}
		return String.valueOf(ch);
	}

	public static void printArray(int[] que) {
		for (int i = 0; i < que.length - 1; i++) {
			System.out.print(que[i] + " ");
		}
		System.out.println(que[que.length - 1]);
	}

	public static void lprintArray(long[] que) {
		for (int i = 0; i < que.length - 1; i++) {
			System.out.print(que[i] + " ");
		}
		System.out.println(que[que.length - 1]);
	}

	public static int[][] doublesort(int[][] a) {
		Arrays.sort(a, (x, y) -> Integer.compare(x[0], y[0]));
		return a;
	}

	public static long[][] ldoublesort(long[][] a) {
		Arrays.sort(a, (x, y) -> Long.compare(x[0], y[0]));
		return a;
	}

	static long modpow(long x, long n, long mo) {
		long sum = 1;
		while (n > 0) {
			if ((n & 1) == 1) {
				sum = sum * x % mo;
			}
			x = x * x % mo;
			n >>= 1;
		}
		return sum;
	}

	public static char[] revch(char ch[]) {
		char ret[] = new char[ch.length];
		for (int i = ch.length - 1, j = 0; i >= 0; i--, j++) {
			ret[j] = ch[i];
		}
		return ret;
	}

	public static int[] revint(int ch[]) {
		int ret[] = new int[ch.length];
		for (int i = ch.length - 1, j = 0; i >= 0; i--, j++) {
			ret[j] = ch[i];
		}
		return ret;
	}

	public static void warshall_floyd(long v[][]) {
		int n = v[0].length;
		for (int k = 0; k < n; k++)
			for (int i = 0; i < n; i++)
				for (int j = 0; j < n; j++)
					v[i][j] = lmin(v[i][j], v[i][k] + v[k][j]);
	}

	public static long modinv(long a, long m) {
		long b = m, u = 1, v = 0;
		while (b != 0) {
			long t = a / b;
			a -= t * b;
			long x = a;
			a = b;
			b = x;
			u -= t * v;
			x = u;
			u = v;
			v = x;
		}
		u %= m;
		if (u < 0)
			u += m;
		return u;
	}

	public static long lmod(long i, long j) {
		return (i % j) < 0 ? (i % j) + 0 + (j < 0 ? -j : j) : (i % j + 0);
	}

	public static int next_combination(int sub) {
		int x = sub & -sub, y = sub + x;
		return (((sub & ~y) / x) >> 1) | y;
	}
	public static void main(String[] $) throws IOException {
		int n=scan.nextInt();
		int m=scan.nextInt();
		List<ArrayList<Integer>>ls=new ArrayList<ArrayList<Integer>>();
		for(int i=0;i<n;i++)ls.add(new ArrayList<Integer>());
		for(int i=0;i<m;i++){
			int x=scan.nextInt()-1;
			int y=scan.nextInt()-1;
			ArrayList<Integer>l=ls.get(x);
			l.add(y);
			ls.set(y, l);
			l=ls.get(y);
			l.add(x);
			ls.set(x, l);
		}
		boolean visited[]=new boolean[n];
		int ans=0;
		for(int i=0;i<n;i++) {
			if(!visited[i]) {
				visited[i]=true;
				ans++;
				Deque<Integer>dq=new ArrayDeque<Integer>();
				dq.add(i);
				while(dq.size()>0){
					int x=dq.poll();
					ArrayList<Integer>l=ls.get(x);
					for(int q:l) {
						if(!visited[q]) {
							visited[q]=true;
							dq.add(q);
						}
					}
				}
			}
		}
		System.out.println(ans-1);
	}
}