To find the sum of the given series, we can group the terms into pairs of positive and negative numbers.

The first pair is 1 + 2 = 3.
The second pair is -3 - 4 = -7.
The third pair is 5 + 6 = 11.
The fourth pair is -7 - 8 = -15.
And so on.

We can observe that the sum of each pair is always 1 less than a multiple of 4.

There are 50 pairs in total, so the sum of the pairs is 50 * (-1) = -50.

However, we need to consider the last term, which is 99 + 100 = 199.

Therefore, the sum of the given series is -50 + 199 = 149.