Score : 400 points
You are given an integer N. Find the number of strings of length N that satisfy the following conditions, modulo 10^9+7:
A, C, G and T.AGC as a substring.A substring of a string T is a string obtained by removing zero or more characters from the beginning and the end of T.
For example, the substrings of ATCODER include TCO, AT, CODER, ATCODER and (the empty string), but not AC.
Input is given from Standard Input in the following format:
N
Print the number of strings of length N that satisfy the following conditions, modulo 10^9+7.
3
61
There are 4^3 = 64 strings of length 3 that do not contain characters other than A, C, G and T. Among them, only AGC, ACG and GAC violate the condition, so the answer is 64 - 3 = 61.
4
230
100
388130742
Be sure to print the number of strings modulo 10^9+7.