A ternary string of length n is a sequence of n digits in which only 0, 1, or 2 appear.
For example, (0,1,0,2) and (1,1,2,2) are ternary strings of length 4 and can be seen as: 0102 and 1122.
How many ternary strings of length 2n are there in which the zeros appear only in odd positions?