using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using static System.Console;
using static System.Math;
using static MyIO;
using static MyUtil;

public class C { public static void Main() => (new Solver()).Solve(); }

public class Solver
{
	public void Solve()
	{
		long N = GetLong();
		long M = GetLong();
		var uf = new UnionFind(N);
		for(long i = 0; i < M; i++)
		{
			long A = GetLong() - 1;
			long B = GetLong() - 1;
			if(!uf.IsSame(A, B))
				uf.Unite(A, B);
		}

		var set = new HashSet<long>();
		for(long i = 0; i < N; i++)
			set.Add(uf.Root(i));

		WriteLine(set.Count - 1);
	}
}

public class UnionFind
{
	private long[] par = null;
	private long[] rank = null;
	private long[] size = null;
	
	public long Size(long x) => size[Root(x)];
	public bool IsSame(long x, long y) => (Root(x) == Root(y));

	public UnionFind(long n)
	{
		par = new long[n];
		rank = new long[n];
		size = new long[n];
		for(long i = 0; i < n; i++)
		{
			par[i] = i;
			rank[i] = 0;
			size[i] = 1;
		}
	}

	public long Root(long x)
	{
		if(par[x] == x)
			return x;

		long r = Root(par[x]);
		par[x] = r;
		return r;
	}
	
	public bool Unite(long x, long y)
	{
		long rx = Root(x);
		long ry = Root(y);
		if(rx == ry)
			return false;
		
		if(rank[rx] < rank[ry])
		{
			long tmp = rx;
			rx = ry;
			ry = tmp;
		}
		else if(rank[rx] == rank[ry])
			rank[rx]++;

		par[ry] = rx;
		size[rx] = size[rx] + size[ry];
		return true;
	}
}

public static class MyIO
{
	private static string[] args = new string[0];
	private static int cnt = 0;
	private static IEnumerable<string> getArgs(long N) => Range(0, N).Select(_ => getArg());
	private static string getArg()
	{
		if(cnt == args.Length) { args = Console.ReadLine().Split(); cnt = 0; }
		return args[cnt++];
	}

	public static int GetInt() => int.Parse(getArg());
	public static long GetLong() => long.Parse(getArg());
	public static double GetDouble() => double.Parse(getArg());
	public static decimal GetDecimal() => decimal.Parse(getArg());
	public static string GetString() => getArg();
	public static char GetChar() => getArg()[0];
	public static int[] GetInts(long N) => getArgs(N).Select(int.Parse).ToArray();
	public static long[] GetLongs(long N) => getArgs(N).Select(long.Parse).ToArray();
	public static double[] GetDoubles(long N) => getArgs(N).Select(double.Parse).ToArray();
	public static decimal[] GetDecimals(long N) => getArgs(N).Select(decimal.Parse).ToArray();
	public static string[] GetStrings(long N) => getArgs(N).ToArray();
}

public static class MyUtil
{
	public static void chmax<T>(ref T x, T y) where T : IComparable<T> { if(x.CompareTo(y) < 0) x = y; }
	public static void chmin<T>(ref T x, T y) where T : IComparable<T> { if(x.CompareTo(y) > 0)	x = y; }
	public static void swap<T>(ref T x, ref T y) { T tmp = x; x = y; y = tmp; }
	public static long rightShift(long x, long y) => (x >> (int)y);
	public static long leftShift(long x, long y) => (x << (int)y);
	public static bool contains(long s, long x) => (rightShift(s, x) & 1L) == 1L;
	public static long gcd(long x, long y) => (y == 0) ? x : gcd(y, x % y);
	public static long lcm(long x, long y) => (x / gcd(x,y)) * y;	
	public static IEnumerable<int> Range(long start, long count) => Enumerable.Range((int)start, (int)count);
	public static void Sort(this List<string> source) => source.Sort(StringComparer.Ordinal);
}


Measured memory_bytes: 43428