public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int p = in . nextInt ( ) , n = in . nextInt ( ) , a [ ] = new int [ p ] , i , o = - 1 ;
  for ( i = 0 ;
  i < n ;
  i ++ ) {
    int x = in . nextInt ( ) ;
    a [ x % p ] ++ ;
    if ( a [ x % p ] > 1 ) {
      o = i + 1 ;
      break ;
    }
  }
  in . close ( ) ;
  System . out . println ( o ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int p = in . nextInt ( ) , n = in . nextInt ( ) , a [ ] = new int [ p ] , i , o = - 1 ; for ( i = 0 ; i < n ; i ++ ) { int x = in . nextInt ( ) ; a [ x % p ] ++ ; if ( a [ x % p ] > 1 ) { o = i + 1 ; break ; } } in . close ( ) ; 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) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier)))) declarator: (variable_declarator name: (identifier)) declarator: (variable_declarator name: (identifier) value: (unary_expression operand: (decimal_integer_literal)))) (for_statement init: (assignment_expression left: (identifier) right: (decimal_integer_literal)) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (identifier) right: (identifier))))) (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (identifier))) right: (decimal_integer_literal))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (binary_expression left: (identifier) right: (decimal_integer_literal)))) (break_statement))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))