public static void main ( String ared [ ] ) {
  Scanner sc = new Scanner ( System . in ) ;
  int test = sc . nextInt ( ) ;
  while ( test -- > 0 ) {
    int n = sc . nextInt ( ) ;
    int b [ ] = new int [ 2 ] ;
    int a [ ] = new int [ n ] ;
    b [ 0 ] = 0 ;
    b [ 1 ] = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      a [ i ] = sc . nextInt ( ) ;
      if ( a [ i ] % 2 != i % 2 ) b [ i % 2 ] ++ ;
    }
    if ( b [ 0 ] == b [ 1 ] ) System . out . println ( b [ 0 ] ) ;
    else System . out . println ( "-1" ) ;
  }
}

public static void main ( String ared [ ] ) { Scanner sc = new Scanner ( System . in ) ; int test = sc . nextInt ( ) ; while ( test -- > 0 ) { int n = sc . nextInt ( ) ; int b [ ] = new int [ 2 ] ; int a [ ] = new int [ n ] ; b [ 0 ] = 0 ; b [ 1 ] = 0 ; for ( int i = 0 ; i < n ; i ++ ) { a [ i ] = sc . nextInt ( ) ; if ( a [ i ] % 2 != i % 2 ) b [ i % 2 ] ++ ; } if ( b [ 0 ] == b [ 1 ] ) System . out . println ( b [ 0 ] ) ; else System . out . println ( " - 1" ) ; } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (type_identifier) name: (identifier) dimensions: (dimensions))) 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)))) (while_statement condition: (parenthesized_expression (binary_expression left: (update_expression (identifier)) right: (decimal_integer_literal))) body: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (decimal_integer_literal))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier))))) (expression_statement (assignment_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (decimal_integer_literal))) (expression_statement (assignment_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (decimal_integer_literal))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal)) right: (binary_expression left: (identifier) right: (decimal_integer_literal)))) consequence: (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal)))))))) (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (array_access array: (identifier) index: (decimal_integer_literal)))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (decimal_integer_literal))))) alternative: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal)))))))))