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It is a standard combinatorics problem. You need to select a pair of people from N people. How many different pairs are there? N choose 2. N! / ((N-2)! * 2!) which simplifies to (N * (N-1)) / 2.
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It is a standard combinatorics problem. You need to select a pair of people from N people. How many different pairs are there? N choose 2. N! / ((N-2)! * 2!) which simplifies to (N * (N-1)) / 2.