Solved it thanks to your comment.

Thank you, this comment should be added to the description

I don't think he was serious with that solution

because it doesn't "consume" what would otherwise be the second group. See:

    test.assert_equals(insert_dash(13579),'1-3-5-7-9')

I think they prefer to avoid it because its easy to get lost in it ....

But it's a nice feature! I really love it

nice outside the box thinking

very elegant, good job

nice one

You have to use only number "1" for the check.

Done.

count the number of 1s in the array

You're not doing that.

Description should be more informative, my code pass sample tests but failed on random input...