The arithmetic module only includes CEIL or FLOOR operations, no ROUND operation.
We can create one using an IF statement.

Let's say we want to round S1 to the closest integer. The function is:

IF(S1<(CEIL(S1)+FLOOR(S1))/2,FLOOR(S1),CEIL(S1))


How does it work?
Example: S1 = 12.7. The expected result for rounding is 13.

CEIL(S1)=13
FLOOR(S1)=12
(CEIL(S1)+FLOOR(S1))/2 = 12.5

A test is done: if S1 is lower than (CEIL(S1)+FLOOR(S1))/2, it will return FLOOR(S1). If not, it returns CEIL(S1).
In the example, the test "12.7 < 12.5" is false, so the function returns CEIL(12.7)=13.

What happens in the 12.5 case?
The function is still correct. "<" means "strictly lower than".
So the test "12.5 < 12.5" is technically False, the function returns CEIL(12.5)=13, which is the expected result.

More information:
IF(S1<(CEIL(S1)+FLOOR(S1))/2,FLOOR(S1),CEIL(S1)) contains exactly 48 characters, which is the limit for the arithmetic module in the meters (limit is 249 in the VIP).
So rounding the S1-S8 inputs is OK, but rounding (S1+S2) or S1/1000 requires an additional arithmetic module.
Check the ION Reference for more details.