Find the number that can replace the question mark in the series given below: 5, 9, 3, 11, 1, 13, -1, ?

The next number is 15.

The series interleaves two sequences by position.

Odd positions 5, 3, 1, -1 drop by 2 each time.

Even positions 9, 11, 13, ? rise by 2 each time.

The question mark sits at an even position, so it is 13 + 2 = 15.

Thus the required value is 15.