Heading ##Total count is even. Missing number (?) is 8th which can be calculated by using logic:
Subtract the middle one's, then their neighbour digits. So, the missing number be:
case1:
12-12 = 0
13-11= 2
15-11= 4
2- (?) = 6
case2:
12-12= 0
11-13= -2
11-15= -4
(?)- 2 = -6
which comes -4 in both the cases.
so, the final series is: 2, 15, 13, 12, 12, 11, 11, -4
