For a sequence a1, a2, ... , an, consider the following operation f: f = {b1, b2, ... , bn-1}, where bi= |ai - ai+1|. After applying the operation f for n - 1 times, denoted by fn-1, the sequence will become a single element. Bobo has a sequence a of length n filled with 0, 1 and ?. He would like to know the number of ways modulo to replace ?to 0 or 1 such that fn-1 = {1}.
For a sequence a1, a2, ... , an, consider the following operation f: f(a) = {b1, b2, ... , bn-1}, where bi = |ai - ai+1|. After applying the operation f for n - 1 times, denoted by fn-1(a), the sequence will become a single element. Bobo has a sequence a of length n filled with 0, 1 and ?. He would like to know the number of ways modulo (109+7) to replace ? to 0 or 1 such that fn-1(a) = {1}.
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