A dec-string is a sequence of characters from the 10-character alphabet {0,1,2,3,4,5,6,7,8,9}. A dec-string d1,..., dn is bad if di = d;+1 or di + d+1 = 9 for at least one E {1,...,n-1} and it is good otherwise. What is the number of good dec-strings of length n