1
Vitsy, 14 11 - 25 = -14 Bytes
Note: This language was made after this question was asked, but it was not created for this task.
2a{b*\[DO];
I'm pretty sure this can be golfed down a little more, but here you go:
2a{b*\[DO];
2 Push 2 to the stack as the backup value - if input is ...