So, I'm trying to use a **for** loop to increment a variable I created called `sum`

with the last digit of the input, then dividing the input by 10 until there's only one digit, then adding it to `sum`

.

Here's my code:

```
def digit_sum(n):
sum = 0
for len(n) > 1:
num = n - int(n/10)*10
sum += num
n = int(n/10)
return sum
```

However, it always gives me the following mistake:

```
File "python", line 3
for len(n) > 1:
^
SyntaxError: invalid syntax
```

What's wrong with my code?