public static void main ( String args [ ] ) throws IOException {
  BufferedReader br = new BufferedReader ( new InputStreamReader ( System . in ) ) ;
  int t = Integer . parseInt ( br . readLine ( ) ) ;
  while ( t -- > 0 ) {
    int n = Integer . parseInt ( br . readLine ( ) ) ;
    int arr [ ] = new int [ n ] ;
    String s [ ] = br . readLine ( ) . split ( " " ) ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) arr [ i ] = Integer . parseInt ( s [ i ] ) ;
    int even = 0 , odd = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      if ( arr [ i ] % 2 != 0 && i % 2 == 0 ) odd ++ ;
      else if ( arr [ i ] % 2 == 0 && i % 2 != 0 ) even ++ ;
    }
    if ( odd == even ) System . out . println ( even ) ;
    else if ( odd == 0 && even == 0 ) System . out . println ( "0" ) ;
    else System . out . println ( "-1" ) ;
  }
}

public static void main ( String args [ ] ) throws IOException { BufferedReader br = new BufferedReader ( new InputStreamReader ( System . in ) ) ; int t = Integer . parseInt ( br . readLine ( ) ) ; while ( t -- > 0 ) { int n = Integer . parseInt ( br . readLine ( ) ) ; int arr [ ] = new int [ n ] ; String s [ ] = br . readLine ( ) . split ( " ▁ " ) ; for ( int i = 0 ; i < n ; i ++ ) arr [ i ] = Integer . parseInt ( s [ i ] ) ; int even = 0 , odd = 0 ; for ( int i = 0 ; i < n ; i ++ ) { if ( arr [ i ] % 2 != 0 && i % 2 == 0 ) odd ++ ; else if ( arr [ i ] % 2 == 0 && i % 2 != 0 ) even ++ ; } if ( odd == even ) System . out . println ( even ) ; else if ( odd == 0 && even == 0 ) System . out . println ( "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))) (throws (type_identifier)) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (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 (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 (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))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) dimensions: (dimensions) value: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) name: (identifier) arguments: (argument_list (string_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: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (identifier))))))) (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: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal)) right: (decimal_integer_literal)) right: (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal)))) consequence: (expression_statement (update_expression (identifier))) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal)) right: (decimal_integer_literal)) right: (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal)))) consequence: (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 (identifier)))) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (binary_expression left: (identifier) right: (decimal_integer_literal)))) 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 (string_literal))))))))))