public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int memberCnt = sc . nextInt ( ) ;
  int relatedCnt = sc . nextInt ( ) ;
  HashMap < Integer , ArrayList < Integer >> relations = new HashMap < Integer , ArrayList < Integer >> ( ) ;
  for ( int i = 0 ;
  i < relatedCnt ;
  i ++ ) {
    int oneSelf = sc . nextInt ( ) ;
    int pair = sc . nextInt ( ) ;
    ArrayList < Integer > friends = new ArrayList < Integer > ( ) ;
    if ( relations . containsKey ( oneSelf ) ) {
      friends = relations . get ( oneSelf ) ;
    }
    friends . add ( pair ) ;
    relations . put ( oneSelf , friends ) ;
  }
  int max = 1 ;
  for ( int n = 1 ;
  n <= memberCnt ;
  n ++ ) {
    if ( relations . containsKey ( n ) ) {
      max = Math . max ( max , 1 + getMaxRelations ( relations , relations . get ( n ) ) ) ;
    }
  }
  System . out . println ( max ) ;
  sc . close ( ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int memberCnt = sc . nextInt ( ) ; int relatedCnt = sc . nextInt ( ) ; HashMap < Integer , ArrayList < Integer > > relations = new HashMap < Integer , ArrayList < Integer > > ( ) ; for ( int i = 0 ; i < relatedCnt ; i ++ ) { int oneSelf = sc . nextInt ( ) ; int pair = sc . nextInt ( ) ; ArrayList < Integer > friends = new ArrayList < Integer > ( ) ; if ( relations . containsKey ( oneSelf ) ) { friends = relations . get ( oneSelf ) ; } friends . add ( pair ) ; relations . put ( oneSelf , friends ) ; } int max = 1 ; for ( int n = 1 ; n <= memberCnt ; n ++ ) { if ( relations . containsKey ( n ) ) { max = Math . max ( max , 1 + getMaxRelations ( relations , relations . get ( n ) ) ) ; } } System . out . println ( max ) ; sc . close ( ) ; }
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