I was reading this article on wikipedia about reorganizing the order of summation of a series. I dont understand why if we reorganise the series
$0=1-1+\frac{1}{2}-\frac{1}{2}+\frac{1}{3}-\frac{1}{3}+\frac{1}{4}-\frac{1}{4}+\ldots$
By taking first $p$ positive numbers of the series and then $q$ negative ones we get $ln(p/q)$ as result.