Nice idea. The difficulty curve should be tuned a bit. For me it stayed too long in the <100 range, but then when it increased to <1000 it was too much (keeping both the speed and introducing so many new numbers). I'd recommend slowing the speed down a bit when increasing the sum, and increasing the sum by 100~200 at a time, instead of a jump from <100 to <1000, if that makes any sense :)