X o o o o o o
x X o o o o o
x x X o o o o
x x x X o o o
x x x x X o o
x x x x x X o
x x x x x x X

##</code><code>##

(n*n - n)/2 + n = n*n/2 - n/2 + n
                = n*n/2 + n/2
                = n*(n+1)/2