В массиве a(n,m) найти среднее значение произвольного столбца — Basic(Бейсик)

INPUT"vvedite chislo strok ",n
INPUT"vvedite chislo stolbcov ",m
DIM a(n,m)
RANDOMIZE TIMER
FOR i=1 TO n
FOR j=1 TO m
a(i,j)=INT(RND*20)
NEXT
NEXT
PRINT "massiv"
FOR i=1 TO n
FOR j=1 TO m
PRINT a(i,j);" ";
NEXT
PRINT
NEXT
DO
INPUT"vvedite nomer stolbca ",k 
LOOP UNTIL k<=m
FOR i=1 TO n
s=s+a(i,k)
NEXT
PRINT"srednee znachenie stolbca ";k"=";s/n