Add, Add, Subtract

What is the value of the following expression where all the whole numbers 1 through 99 are included and the repeated operations are add, add, subtract?

0 + 1 + 2 – 3 + 4 + 5 – 6 + 7 + 8 – 9 + . . . + 97 + 98 – 99

(Source: Adapted from Mathematics Teaching in the Middle School, May 1997)