QBASIC to find Sum of digits using SUB

DECLARE SUB SUM ( N )
CLS
INPUT "Enter any number"; N
CALL SUM ( N )
END

SUB SUM ( N )
S = 0
WHILE N <> 0
R = N MOD 10
S = S + R
N = N \ 10
WEND
PRINT "Sum of digits="; S
END SUM