Complete the series: 15, 13, 11, 14, 17, _____?

Complete the series: 15, 13, 11, 14, 17, _____?

Explanation

The pattern appears to be a repeating sequence of:

-2, -2, +3, +3, -4, -4

15 (-2) = 13

13 (-2) = 11

11 (+3) = 14

14 (+3) = 17

17 (-4) = 13