public static void main ( String [ ] args ) {
  try ( java . util . Scanner sc = new java . util . Scanner ( System . in ) ) {
    int n = sc . nextInt ( ) ;
    int m = sc . nextInt ( ) ;
    int [ ] [ ] mm = new int [ n ] [ m ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      String [ ] aa = sc . next ( ) . split ( "" ) ;
      for ( int j = 0 ;
      j < aa . length ;
      j ++ ) {
        mm [ i ] [ j ] = Integer . parseInt ( aa [ j ] ) ;
      }
    }
    boolean isOK = true ;
    for ( int i = 1 ;
    i < n ;
    i ++ ) {
      for ( int j = 1 ;
      j < m ;
      j ++ ) {
        if ( mm [ i - 1 ] [ j ] == mm [ i ] [ j ] ) {
          isOK = false ;
        }
      }
    }
    if ( isOK ) {
      for ( int i = 0 ;
      i < n ;
      i ++ ) {
        for ( int j = 1 ;
        j < m ;
        j ++ ) {
          if ( mm [ i ] [ j - 1 ] != mm [ i ] [ j ] ) {
            isOK = false ;
          }
        }
      }
    }
    System . out . println ( isOK ? "YES" : "NO" ) ;
  }
}

public static void main ( String [ ] args ) { try ( java . util . Scanner sc = new java . util . Scanner ( System . in ) ) { int n = sc . nextInt ( ) ; int m = sc . nextInt ( ) ; int [ ] [ ] mm = new int [ n ] [ m ] ; for ( int i = 0 ; i < n ; i ++ ) { String [ ] aa = sc . next ( ) . split ( " " ) ; for ( int j = 0 ; j < aa . length ; j ++ ) { mm [ i ] [ j ] = Integer . parseInt ( aa [ j ] ) ; } } boolean isOK = true ; for ( int i = 1 ; i < n ; i ++ ) { for ( int j = 1 ; j < m ; j ++ ) { if ( mm [ i - 1 ] [ j ] == mm [ i ] [ j ] ) { isOK = false ; } } } if ( isOK ) { for ( int i = 0 ; i < n ; i ++ ) { for ( int j = 1 ; j < m ; j ++ ) { if ( mm [ i ] [ j - 1 ] != mm [ i ] [ j ] ) { isOK = false ; } } } } System . out . println ( isOK ? " YES " : " NO " ) ; } }
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