text: Let call A, B, C and D the 4 first people E,F,G the three others and M the manager.\nThe communication channels are :\nA-B, A-C, A-D, B-C, B-D, C-D, A-M, B-M, C-M and D-M\nso 10 in total\nIf you add 3 people E, F,G you have added now :\nA-E, A-F, A-G, B-E, B-F, B-G, C-E, C-F, C-G, D-E, D-F, D-G, E-F, E-G, F-G, M-E, M-F, M-G\nso 18 added\nand 28 in total\n\nIt is also how to select 2 people between n :\nC(2,n) = n (n-1)/2 = 8*7/2 = 28