If you try to make a kata with restrictions, you need to do hard, harder, even harder

do not use any loop techniks.

And you did not even try to ban for or while.
Are they not loops to you?

Use Test.assertEquals(actual, expected[, message]) instead of Test.expect to provide friendly debug message.

Needs Random Tests.

Sum the input has been done millions of times, and nope, adding restrictions does not make it different.