We know that every router in this network will form an adjacency with DR and BDR. and we will have one adjacency between DR and BDR as well.
so the formula for this could be.
n-2 (removed DR and BDR from the total n routers).
now these routers will form adjacency with DR and BDR both
(n-2)+(n-2) and we have to add one more adjacency between DR and BDR. so
(n-2)+(n-2)+1=2n-3 is the numbers
Example:
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2 routers === 2*2-3=1 adjacency
4 routers === 2*4-3=5 adjacency.
