Hello thereddevilsDo you know how to find the sum of an arithmetic progression (AP)? One formula (there are other versions of it) you can use is

where and are the first and last terms, and is the number of terms.

Now we want the sum of all the integers between and inclusive. So that is

And this is an AP where and the number of terms is

So plug these values into the formula above, and the sum is

Grandad