Дан массив D(n), n — кратен 3. Поменять местами 1 и 3 элементы, 4 и 6, 7 и 9 и т.д — Pascal(Паскаль)

Program fail1;

label 1;

var
  i, n: integer;
  m: array [1 .. 1000] of integer;

procedure exchange(var a, b: integer);
var
  c: integer;
begin
  c := a;
  a := b;
  b := c;
end;

begin
1:
  writeln('Vvedite N, kratnoe 3: ');
  readln(n);
  if n mod 3 <> 0 then
  begin
    writeln('Vi vveli cislo NE kratnoe 3. Povtorite popitku...');
    goto 1;
  end;
  writeln('Elementi massiva:');
  for i := 1 to n do
    m[i] := i;
  for i := 1 to n do
    write(m[i], ' ');

  i := 1;
  repeat
    exchange(m[i], m[i + 2]);
    i := i + 3;
  until i > n;
  writeln;
  writeln('Ishodnii massiv:');
  for i := 1 to n do
    write(m[i], ' ');
  readln;

end.