public static void main ( String [ ] args ) {
  int N = sc . nextInt ( ) ;
  int M = sc . nextInt ( ) ;
  UnionFind uf = new UnionFind ( N ) ;
  uf . init ( N ) ;
  for ( int i = 0 ;
  i < M ;
  i ++ ) {
    int a = sc . nextInt ( ) - 1 ;
    int b = sc . nextInt ( ) - 1 ;
    uf . unite ( a , b ) ;
  }
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    uf . find ( i ) ;
  }
  boolean [ ] used = new boolean [ N ] ;
  int ans = - 1 ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    if ( ! used [ uf . parent [ i ] ] ) {
      used [ uf . parent [ i ] ] = true ;
      ans ++ ;
    }
  }
  System . out . println ( ans ) ;
}

public static void main ( String [ ] args ) { int N = sc . nextInt ( ) ; int M = sc . nextInt ( ) ; UnionFind uf = new UnionFind ( N ) ; uf . init ( N ) ; for ( int i = 0 ; i < M ; i ++ ) { int a = sc . nextInt ( ) - 1 ; int b = sc . nextInt ( ) - 1 ; uf . unite ( a , b ) ; } for ( int i = 0 ; i < N ; i ++ ) { uf . find ( i ) ; } boolean [ ] used = new boolean [ N ] ; int ans = - 1 ; for ( int i = 0 ; i < N ; i ++ ) { if ( ! used [ uf . parent [ i ] ] ) { used [ uf . parent [ i ] ] = true ; ans ++ ; } } System . out . println ( ans ) ; }
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