Length 1 String -- a,b --(2)
Length 2 String -- aa,bb --(2)
Length 3 String -- aaa,bbb,aba,bab -- (4)
Length 4 String -- aaaa,bbbb,abba,baab ---(4)
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Similarly following pattern till Length 10
We get ==> 2*2 +2*4 +2*8 +2*16 +2*32
==> 4+ 8+16+32+64
==> 124
Ans :124