public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int N = sc . nextInt ( ) ;
  int M = sc . nextInt ( ) ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) node [ i ] = new Node ( ) ;
  for ( int i = 0 ;
  i < M ;
  i ++ ) {
    int a = sc . nextInt ( ) ;
    int b = sc . nextInt ( ) ;
    node [ a - 1 ] . to . add ( b - 1 ) ;
    node [ b - 1 ] . to . add ( a - 1 ) ;
  }
  int count = 0 ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    if ( node [ i ] . visit ) continue ;
    node [ i ] . visit = true ;
    solve ( node [ i ] ) ;
    count ++ ;
  }
  System . out . println ( count - 1 ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int N = sc . nextInt ( ) ; int M = sc . nextInt ( ) ; for ( int i = 0 ; i < N ; i ++ ) node [ i ] = new Node ( ) ; for ( int i = 0 ; i < M ; i ++ ) { int a = sc . nextInt ( ) ; int b = sc . nextInt ( ) ; node [ a - 1 ] . to . add ( b - 1 ) ; node [ b - 1 ] . to . add ( a - 1 ) ; } int count = 0 ; for ( int i = 0 ; i < N ; i ++ ) { if ( node [ i ] . visit ) continue ; node [ i ] . visit = true ; solve ( node [ i ] ) ; count ++ ; } System . out . println ( count - 1 ) ; }
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