#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <type_traits>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std;
using namespace placeholders;

using LL = long long;
using ULL = unsigned long long;
using VI = vector< int >;
using VVI = vector< vector< int > >;
using VS = vector< string >;
using ISS = istringstream;
using OSS = ostringstream;
using PII = pair< int, int >;
using VPII = vector< pair< int, int > >;
template < typename T = int > using VT = vector< T >;
template < typename T = int > using VVT = vector< vector< T > >;
template < typename T = int > using LIM = numeric_limits< T >;
template < typename T = int > using OSI = ostream_iterator< T >;

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }

void in_impl(){};
template < typename T, typename... TS > void in_impl( T &head, TS &... tail ){ cin >> head; in_impl( tail ... ); }
#define IN( T, ... ) T __VA_ARGS__; in_impl( __VA_ARGS__ );

template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; }
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); }

#define NUMBERED( name, number ) NUMBERED2( name, number )
#define NUMBERED2( name, number ) name ## _ ## number
#define REP1( n ) REP2( NUMBERED( REP_COUNTER, __LINE__ ), n )
#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2, REP1 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &&e : c )
#define ALL( c ) begin( c ), end( c )
#define AALL( a ) ( remove_all_extents< decltype( a ) >::type * )a, ( remove_all_extents< decltype( a ) >::type * )a + sizeof( a ) / sizeof( remove_all_extents< decltype( a ) >::type )

#define SZ( v ) ( (int)( v ).size() )
#define EXISTS( c, e ) ( ( c ).find( e ) != ( c ).end() )

template < typename T > inline bool chmin( T &a, const T &b ){ if ( b < a ) { a = b; return true; } return false; }
template < typename T > inline bool chmax( T &a, const T &b ){ if ( a < b ) { a = b; return true; } return false; }

#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

//     Λ   Λ＿_
//  ／(*ﾟーﾟ)／＼
// ／| ￣U U￣|＼／
//   |      |／

class DisjointSetForest
{
private:
	const int N;

	vector<int> parent;
	vector<int> rank;

	int groups_;
	std::vector< int > sizes;

public:
	DisjointSetForest( int n ) : N( n ), parent( N ), rank( N, 0 ), groups_( N ), sizes( N, 1 )
	{
		std::iota( parent.begin(), parent.end(), 0 );

		return;
	}

	int find( int x )
	{
		if ( parent[x] == x )
		{
			return x;
		}
		return parent[x] = find( parent[x] );
	}

	bool same( int x, int y )
	{
		return find( x ) == find( y );
	}

	bool unite( int x, int y )
	{
		x = find( x );
		y = find( y );

		if ( x == y )
		{
			return false;
		}

		if ( rank[x] < rank[y] )
		{
			parent[x] = y;
			sizes[y] += sizes[x];
		}
		else
		{
			parent[y] = x;
			sizes[x] += sizes[y];

			if ( rank[x] == rank[y] )
			{
				++rank[x];
			}
		}

		--groups_;
		return true;
	}

	int groups() const
	{
		return groups_;
	}

	int groupSize( const int x )
	{
		return sizes[ find( x ) ];
	}
};
// DisjointSetForest( N )
// find( x )
// same( x, y )
// unite( x, y )
// groups()
// groupSize( x )

int main()
{
	cin.tie( nullptr );
	ios::sync_with_stdio( false );
	cout << setprecision( 12 ) << fixed;

	IN( int, N, M );
	VI A( M ), B( M );
	REP( i, M )
	{
		cin >> A[i] >> B[i];
		--A[i], --B[i];
	}

	DisjointSetForest dsf( N );
	REP( i, M )
	{
		dsf.unite( A[i], B[i] );
	}

	cout << dsf.groups() - 1 << endl;

	return 0;
}

Measured memory_bytes: 5312