suppose u paired 4,5 what about 12 to pair up 12 again u need to pair it with 4 no other possible way
so u have to pair as 4,5 and 4,12 why to pair 4 twice as we have other choice to pair
same as 4,5 1,9 is paired then 11 will be alone
so again it has to be paired with 9 no other way so we try to pair just once if we have choice