Problem #343
Stringer
Given a string of length n made up of 5 characters a, b, c, d or e.
Let S(n) denote the number of strings of length n such that any of ab, ac, ad, eb, ec or ed doesn't occur as a substring in the string of size n.
Calculate \sum\limits_{i=1}^{10^{18}} S(i) \mod 1000000007