And you can say C could shake A's hand, C could shake B's hand, C could shake, C could shake D's hand. and you could say D could shake A's hand, D could shake B's hand, D could shake C's hand. And this is 12 right over here, and this is a permutations. If D shaking C's hand was actually different than C shaking D's hand, then we would count 12. But we just wanted to say, well, how many ways, they just have to meet each other once. And so we're double counting. So, A, B is the same thing as B, A.