Заполнить квадратный массив змейкой- Basic(Бейсик)

INPUT "vvedite razmer N ", n
DIM a(n, n)
i = n
j = 1
s = -1
DO
DO
k = k + 1
a(i, j) = k
i = i + s
j = j + s
LOOP UNTIL i > n OR j > n OR i < 1 OR j < 1
IF i < 1 AND j < 1 THEN i = 1: j = 2
IF i > n AND j > n THEN i = n - 1: j = n
IF j < 1 THEN j = 1
IF i < 1 THEN i = 1: j = j + 2
IF i > n THEN i = n:
IF j > n THEN i = i - 2: j = n
s = -s
LOOP UNTIL k = n * n
PRINT "massiv"
FOR i = 1 TO n
FOR j = 1 TO n
PRINT a(i, j);
NEXT
PRINT
NEXT