The sum of this series can be calculated by grouping the numbers in pairs and then subtracting the second number from the first number in each pair.

(99-97) + (95-93) + (91-89) + ... + (11-9) + (7-5) + (3-1)
= 2 + 2 + 2 + ... + 2 + 2 + 2
= 2 (49 pairs)
= 2 49
= 98

Therefore, the sum of the series is 98.