Using numbers on Mac or IOS

This is what I would like the formula to do.

If value in A1 is / this goes in A2

1 / 12
2 / 10
3 / 9
4 / 8
5 / 7
6 / 6
7 / 5
8 / 4
9 / 3
10 / 2
11 or higer / 1