public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int n = in . nextInt ( ) , k = in . nextInt ( ) , i , a [ ] = new int [ 26 ] , m = 0 ;
  String s = in . next ( ) ;
  in . close ( ) ;
  for ( i = 0 ;
  i < n ;
  i ++ ) {
    a [ s . charAt ( i ) - 97 ] ++ ;
    m = Math . max ( m , a [ s . charAt ( i ) - 97 ] ) ;
  }
  System . out . println ( m <= k ? "YES" : "NO" ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int n = in . nextInt ( ) , k = in . nextInt ( ) , i , a [ ] = new int [ 26 ] , m = 0 ; String s = in . next ( ) ; in . close ( ) ; for ( i = 0 ; i < n ; i ++ ) { a [ s . charAt ( i ) - 97 ] ++ ; m = Math . max ( m , a [ s . charAt ( i ) - 97 ] ) ; } System . out . println ( m <= k ? " YES " : " NO " ) ; }
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