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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            new ABC122().SolveD(new ConsoleInput(Console.In, ' '));
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }
  class ABC122
    {
        public void SolveD(ConsoleInput cin)
        {
            var n = cin.ReadInt;
            var MOD = (int)(Pow(10, 9) + 7);
            var dp = new int[n + 1, 4, 4, 4];
            //T=0 A=1 G=2 C=3
            //NG: AGC,ACG,GAC,AxGC,AGxC
            //NG: 123,132,213,1x23,12x3
            dp[0, 0, 0, 0] = 1;
            //DP法
            for (int i = 0; i < n; i++)
            {
                for (int a = 0; a < 4; a++)
                {
                    for (int b = 0; b < 4; b++)
                    {
                        for (int c = 0; c < 4; c++)
                        {
                            for (int d = 0; d < 4; d++)
                            {
                                //３文字のNGパターン
                                if (ng(b, c, d) || ng(b, d, c) || ng(c, b, d)) continue;
                                //４文字のNGパターン
                                if (ng(a, c, d) || ng(a, b, d)) continue;
                                dp[i + 1, b, c, d] = (dp[i + 1, b, c, d] + dp[i, a, b, c]) % MOD;
                            }
                        }
                    }
                }
            }
            var sum = 0;

            for (int a = 0; a < 4; a++)
            {
                for (int b = 0; b < 4; b++)
                {
                    for (int c = 0; c < 4; c++)
                    {
                        sum = (sum + dp[n, a, b, c]) % MOD;
                    }
                }
            }

            WriteLine(sum);
            ReadLine();
        }

        bool ng(int a, int b, int c)
        {
            return (a == 1 && b == 2 && c == 3);
        }
    }
}