(1+2+3+4+.....+n)-(2+4+....+n+1)-1=n(n+1)/2-2(1+2+...n+1/2)-1
=n(n+1)/2-2*[(n+1)/2*(n+2)/2]-1=n(n+1)/2-2*(n+1)(n+2)/4. -1
=[2(n+1)-2(n+1)(n+2)]/4-1
=2(n+1)(1-n-2)/4. -1
=-(2n+2)(n+3)/4-1
-(n+1)(n+3)/2 -1
etc
sper sa fie oricum asta e ideea-aplici Gauss
