Visual Basic - como sumar las diagonales de una matriz

[Tarde pero para otros]

program diagonales;

  uses

     crt;

 const

     n=3;

  type

     matriz=array[1..n,1..n] of integer;

  var

     a : matriz;

     suma1,suma2, resta, i, j : integer;

     x, y : integer;

  begin

    clrscr;

    textcolor(15);

    gotoxy(2,2);write('ÚÄÄÄÂÄÄÄÂÄÄÄ¿');

    gotoxy(2,3);write('³   ³   ³   ³');

    gotoxy(2,4);write('ÃÄÄÄÅÄÄÄÅÄÄÄ´');

    gotoxy(2,5);write('³   ³   ³   ³');

    gotoxy(2,6);write('ÃÄÄÄÅÄÄÄÅÄÄÄ´');

    gotoxy(2,7);write('³   ³   ³   ³');

    gotoxy(2,8);write('ÀÄÄÄÁÄÄÄÁÄÄÄÙ');

    x := 3;

    y := 1;

       for j := 1 to n do

       begin

        y := y + 2;

        x := 4;

        for i := 1 to n do

       begin

         gotoxy(3,1);write('introduzca los valores de la matriz :',i,',',j);

         gotoxy(x,y);readln(a[i,j]);

         gotoxy(x,y);write(a[i,j]);

         x := x + 4;

      end;

    end;

    {comienza el ciclo de repeticion}

    gotoxy(2,10);write(' ');

   j := 1;

   suma1 := 0;

  begin

    i := 1;

  repeat

    suma1 := suma1 + a[i,j];

    i := i + 1;

    j := j + 1;

  until i > 3;

    i := 3;

    j := 1;

    suma2 := 0;

  repeat

      suma2 := suma2 + a[i,j];

      i := i - 1;

      j := j + 1;

  until j > 3;

       resta := suma2 - suma1;

  end;

    gotoxy(2,12);write('la suma de la diagonal principal es   : ',suma1);

    gotoxy(2,13);write('la suma de la diagonal secundaria es  : ',suma2);

    gotoxy(2,14);write('la diferencia de las dos diagonales es: ', resta);

    readln;

  end.