Sum of digits of a number using User Defined Function (UDF)

DECLARE FUNCTION digitsum(n)
CLS
INPUT "Enter testing number=";v
PRINT "Sum of ";n;" is "; digitsum(v)
END

FUNCTION digitsum(n)
a=n
WHILE NOT a=0        'writing NOT a=0 is same as writing a<>0
  r=n MOD 10
  s=s+r
  n=int(n/10)
WEND
digitsum=s
END FUNCTION