public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int n = in . nextInt ( ) , i ;
  String s = in . next ( ) , o = "NO" ;
  in . close ( ) ;
  for ( i = 0 ;
  i < n - 1 ;
  i ++ ) if ( s . charAt ( i ) != s . charAt ( i + 1 ) ) {
    o = "YES\n" + s . substring ( i , i + 2 ) ;
    break ;
  }
  System . out . println ( o ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int n = in . nextInt ( ) , i ; String s = in . next ( ) , o = " NO " ; in . close ( ) ; for ( i = 0 ; i < n - 1 ; i ++ ) if ( s . charAt ( i ) != s . charAt ( i + 1 ) ) { o = " YES \n " + s . substring ( i , i + 2 ) ; break ; } System . out . println ( o ) ; }
(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: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) declarator: (variable_declarator name: (identifier))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) declarator: (variable_declarator name: (identifier) value: (string_literal))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) (for_statement init: (assignment_expression left: (identifier) right: (decimal_integer_literal)) condition: (binary_expression left: (identifier) right: (binary_expression left: (identifier) right: (decimal_integer_literal))) update: (update_expression (identifier)) body: (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (identifier) right: (decimal_integer_literal)))))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (binary_expression left: (string_literal) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier) (binary_expression left: (identifier) right: (decimal_integer_literal))))))) (break_statement)))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))