Progessive terms adding and minusing What is the value of
$$1+2+3-4+5+6+7-8+9+10+11+12...+97+98+99-100 \ ?$$
Any help is appreciated, thank you!
I added the terms as an AP then subtracted 10 then all the numbers that were missed out, not sure if this is right though.

If you rearrange the terms, you can write it as:
$1+2+3+5+6+7+9+10+...+98+99-4-8-...-100=1+2+3+4+...+100-2(4+8+12+...+100)= 
1+2+...+100-2\cdot 4(1+2+...+25)=\frac{100(100+1)}{2}-2\cdot4\frac{25(25+1)}{2}$.
