Question 99
For positive integers $n$, $m$, $k$, let us write $n \equiv m \pmod k$ if $n - m$ is divisible by $k$. Let $A$ be the sum of all positive integers $a$ less than or equal to $2011$ for which $a \equiv 1 \pmod 3$, and $B$ be the sum of all positive integers $b$ less than or equal to $2011$ for which $b \equiv 2 \pmod 3$. Find the value of $A - B$.