Profit is not the same as the money he haves. Profit is how much he does of a said deal. It's like this:
-7 +8 -9 +10 = 2
That's the profit. Assuming he had 7 dollars in the beggining, the profit would still be two:
7 - 7 = 0
0 + 8 = 8
8 - 9 = -1
-1 + 10 = 9
9 - 7 (the money he had in the beginning, which is not considered profit) = 2
Or assuming he had 8:
8 - 7 = 1
1 + 8 = 9
9 - 9 = 0
0 + 10 = 10
10 - 8 (the money he had in the beginning, which is not considered profit) = 2
Equation can be also done in this way, considered by some more difficult:
1. Buy stuff for 7
2. Sell stuff for 8
3. Buy stuff for 9
4. Sell stuff for 10
5. ??????
6. PROFIT! (2 dollars)
Flow -"The important is to pwn other ppl"