public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  boolean check = true ;
  int n , m , index = 0 ;
  n = in . nextInt ( ) ;
  m = in . nextInt ( ) ;
  String [ ] a = new String [ n ] ;
  int integer [ ] = new int [ n ] ;
  while ( n != 0 ) {
    a [ index ] = in . next ( ) ;
    for ( int i = 0 ;
    i < a [ index ] . length ( ) - 1 ;
    i ++ ) {
      if ( a [ index ] . charAt ( i ) != a [ index ] . charAt ( i + 1 ) ) {
        check = false ;
        break ;
      }
    }
    index ++ ;
    n -- ;
  }
  if ( check ) {
    ConvertStringToInteger ( a , integer ) ;
    for ( int i = 1 ;
    i < integer . length - 1 ;
    i ++ ) {
      if ( ! ( integer [ i - 1 ] != integer [ i ] && integer [ i ] != integer [ i + 1 ] ) ) {
        check = false ;
        break ;
      }
    }
  }
  if ( check ) {
    System . out . println ( "YES" ) ;
  }
  else {
    System . out . println ( "NO" ) ;
  }
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; boolean check = true ; int n , m , index = 0 ; n = in . nextInt ( ) ; m = in . nextInt ( ) ; String [ ] a = new String [ n ] ; int integer [ ] = new int [ n ] ; while ( n != 0 ) { a [ index ] = in . next ( ) ; for ( int i = 0 ; i < a [ index ] . length ( ) - 1 ; i ++ ) { if ( a [ index ] . charAt ( i ) != a [ index ] . charAt ( i + 1 ) ) { check = false ; break ; } } index ++ ; n -- ; } if ( check ) { ConvertStringToInteger ( a , integer ) ; for ( int i = 1 ; i < integer . length - 1 ; i ++ ) { if ( ! ( integer [ i - 1 ] != integer [ i ] && integer [ i ] != integer [ i + 1 ] ) ) { check = false ; break ; } } } if ( check ) { System . out . println ( " YES " ) ; } else { System . out . println ( " NO " ) ; } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))))) (local_variable_declaration type: (boolean_type) declarator: (variable_declarator name: (identifier) value: (true))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier)) declarator: (variable_declarator name: (identifier)) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (array_type element: (type_identifier) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (type_identifier) dimensions: (dimensions_expr (identifier))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier))))) (while_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))) body: (block (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (binary_expression left: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list (identifier))) right: (method_invocation object: (array_access array: (identifier) index: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (identifier) right: (decimal_integer_literal)))))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (false))) (break_statement))))) (expression_statement (update_expression (identifier))) (expression_statement (update_expression (identifier))))) (if_statement condition: (parenthesized_expression (identifier)) consequence: (block (expression_statement (method_invocation name: (identifier) arguments: (argument_list (identifier) (identifier)))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (binary_expression left: (field_access object: (identifier) field: (identifier)) right: (decimal_integer_literal))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (unary_expression operand: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (array_access array: (identifier) index: (identifier))) right: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal)))))))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (false))) (break_statement))))))) (if_statement condition: (parenthesized_expression (identifier)) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))))