Explanation

• The series alternates between subtracting 1 and adding 2:
  • 12 → 11 (subtract 1)
  • 11 → 13 (add 2)
  • 13 → 12 (subtract 1)
  • 12 → 14 (add 2)
  • 14 → 13 (subtract 1)

Following this alternating pattern, the next step should be adding 2 to 13:

$$13 + 2 = 15$$