When n is 2, we get 1. When n is 3, we get 3. When n is 4, we get 6. When n is 5, we get 10

When n = 2, it is 1 / 2 of n;
When n = 3, it is 2 / 2 times of n;
When n = 4, it is 3 / 2 times of n;
When n = 5, it is 4 / 2 times of n;
.
By analogy
When n = m, it is (m-1) / 2 of n;
When n = n, it is (n-1) / 2 of n
So the formula is n (n-1) / 2

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