### 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**