Write out all the numbers 1...N in order. Write them all out in reverse order. Ann the rows together. Obviously the total of each colum is (N+1) and the full total is Nx(N+1). But we want half that. N/2x(N+1). Issybird has the right answer.
Code:
1 2 3 .... (N-2) (N-1) N
N (N-1) (N-2) .... 3 2 1
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(N+1) (N+1) (N+1) .... (N+1) (N+1) (N+1)
HTH