Problem:
Score : 400 points

Problem Statement

You are given an integer N. Find the number of strings of length N that satisfy the following conditions, modulo 10^9+7:

The string does not contain characters other than A, C, G and T.

The string does not contain AGC as a substring.

The condition above cannot be violated by swapping two adjacent characters once.

Notes

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.

Constraints

3 \leq N \leq 100

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of strings of length N that satisfy the following conditions, modulo 10^9+7.

Sample Input 1

3

Sample Output 1

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.

Sample Input 2

4

Sample Output 2

230

Sample Input 3

100

Sample Output 3

388130742

Be sure to print the number of strings modulo 10^9+7.

Solution in Rust (1.15.1) (judge verdict: Accepted)

 use std::vec::Vec;
use std::collections::VecDeque;
use std::io::prelude::*;
use std::io::stdin;
use std::str::FromStr;

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}

fn main() {
  let MOD: u64 = 1_000_000_000 + 7;
  let n: usize = read();
  let mut dp = vec![vec![vec![vec![0; 4]; 4]; 4]; n+1];
  // 長さ０の文字列は１
  dp[0][3][3][3] = 1;
  // 文字列の文字数
  for len in 0..n {
    // 最後から１文字目の文字
    for i in 0..4 {
      // 最後から２文字目の文字
      for j in 0..4 {
        // 最後から3文字目の文字
        for k in 0..4 {
          // 条件に当てはまる物が無い場合はcontinue
          if dp[len][i][j][k] == 0 { continue; }

          // 新しく追加する文字
          for a in 0..4 {
            // ダメな５つの条件をカットする
            if a == 2 && i == 1 && j == 0 { continue; }
            if a == 2 && i == 0 && j == 1 { continue; }
            if a == 1 && i == 2 && j == 0 { continue; }
            if a == 2 && i == 1 && k == 0 { continue; }
            if a == 2 && j == 1 && k == 0 { continue; }

            // ダメな条件を抜けたら、aを足した文字列が作れる
            dp[len + 1][a][i][j] += dp[len][i][j][k];
            dp[len + 1][a][i][j] %= MOD;
          }
        }
      }
    }
  }

  let mut ans = 0;

  for i in 0..4 {
    for j in 0..4 {
      for k in 0..4 {
        ans += dp[n][i][j][k];
        ans %= MOD;
      }
    }
  }

  println!("{}", ans);
}