public static void main ( String args [ ] ) {
  Scanner sc = new Scanner ( System . in ) ;
  int t = sc . nextInt ( ) ;
  while ( t > 0 ) {
    int n = sc . nextInt ( ) ;
    int ar [ ] = new int [ n ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      ar [ i ] = sc . nextInt ( ) ;
    }
    int odd = 0 , even = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      if ( ar [ i ] % 2 != i % 2 ) {
        if ( ar [ i ] % 2 == 1 ) odd ++ ;
        else even ++ ;
      }
    }
    if ( odd != even ) System . out . println ( "-1" ) ;
    else System . out . println ( even ) ;
    t -- ;
  }
}

public static void main ( String args [ ] ) { Scanner sc = new Scanner ( System . in ) ; int t = sc . nextInt ( ) ; while ( t > 0 ) { int n = sc . nextInt ( ) ; int ar [ ] = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) { ar [ i ] = sc . nextInt ( ) ; } int odd = 0 , even = 0 ; for ( int i = 0 ; i < n ; i ++ ) { if ( ar [ i ] % 2 != i % 2 ) { if ( ar [ i ] % 2 == 1 ) odd ++ ; else even ++ ; } } if ( odd != even ) System . out . println ( " - 1" ) ; else System . out . println ( even ) ; t -- ; } }
(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: (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 (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: (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)))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal)) declarator: (variable_declarator name: (identifier) value: (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 (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: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (expression_statement (update_expression (identifier))) alternative: (expression_statement (update_expression (identifier)))))))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) consequence: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal)))) alternative: (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))) (expression_statement (update_expression (identifier)))))))