The sum could be either a 3, a 5, a 7, or an 11.
3 = (1,2) or (2,1)
5 = (1,4), (4,1), (2,3) or (3,2)
7 = (1,6), (6,1), (2,5), (5,2), (3,4), or (4,3)
11 = (5,6) or (6,5)
There are 14 winning possibilities out of a possible 36 possibilities: 14/36 = 7/18