Take two numbers from these numbers, and the total is the combined number (n+ 1, 2). When the absolute value of n(n+ 1)/2 is 1, there are n possible probabilities P 1=2n/[n(n+ 1)] When the absolute value is 2, there are n- 1 possible probabilities p2 = When the absolute value is 3, there are n-2 possible probabilities P3=2(n-2)/[n(n+ 1)]. ................................. (I believe you can see the law) When the absolute value is n- 1, there are two possible probabilities pn-1= 2 * 2/[n (n+1)] When the absolute value is n, There are 1 possible probabilities pn = 2 *1[n (n+1)], so the total expected value is 2 [n+2 (n- 1)+3 (n) because 2 ∑ (k =/kloc-)