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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

/*
......................................................................................................................................
.....................................  ________ ____ __________________________________________ .....................................
..................................... /  _____/|    |   \      \__    ___/\_   _____/\______   \.....................................
...................................../   \  ___|    |   /   |   \|    |    |    __)_  |       _/.....................................
.....................................\    \_\  \    |  /    |    \    |    |        \ |    |   \.....................................
..................................... \______  /______/\____|__  /____|   /_______  / |____|_  /.....................................
.....................................        \/                \/                 \/         \/ .....................................
......................................................................................................................................
.............................................................,;'';:...................................................................
........................................................+@@@@@@@@@@@@@@'..............................................................
.....................................................#@@@##############@@@:...........................................................
...................................................@@@####################@@,.........................................................
.................................................@@#########################@@........................................................
...............................................:@############################@@.......................................................
..............................................@@######@@@#';;'#@@@############@@:.....................................................
.............................................@#####@@,````````````,@@###########@:....................................................
............................................@####@;``````````````````+@##########@....................................................
...........................................@###@:``````````````````````#@########@@...................................................
..........................................@####``````````````````````````@########@@..................................................
.........................................###@.````````````````````````````@########@+.................................................
.........................................@#@```````````````````````````````#########@.................................................
........................................@#@`````````````````````````````````########@@................................................
.......................................,@@```````````````````````````````````@#######@:...............................................
.......................................@@`````````````````````````````````````@#######@...............................................
.......................................@:````````````````````#@@'``````````````@######@+..............................................
......................................#@```````````````````@@@@@@@#````````````########@..............................................
......................................@```````````````````@@@@@@@@@@````````````@######@+.............................................
......................................@``````````````````@@@@@@@+   +```````````+#######@.............................................
.....................................;:``````````````````@@@@@@@    @````````````@######@'............................................
.....................................@``````````````````:@@@@@@@    @````````````@#######@............................................
.....................................@```,@@@#``````````;@@@@@@@    @````````````:#######@:...........................................
.....................................@``@@@@@@@@````````.@@@@@@@#  ,#`````````````@#######@...........................................
.....................................@`@@@@@@@+'@````````@@@@@@@@@@@``````````````@#######@...........................................
.....................................@,@@@@@@   ,```:+:``:@@@@@@@@@.``````````````@########@..........................................
.....................................#@@@@@@@  ;@@#;,,,@``:@@@@@@@````````````````#########@..........................................
.....................................+@@@@@@@@',,,,,,,,;,```.'+;``````````````````'########@;.........................................
.....................................'@@@@',,,,,,,,,,,,,@`````````````````````````:#########@.........................................
....................................:@#,,,,,:;;;;;:,,,,,@`````````````````````````.#########@.........................................
.................................:@#@@@@#++';;;;;;;;;;;;@``````````````````````````##########+........................................
...............................#@#+;;;;;;;;;;;;;;;;;;;;':``````````````````````````##########@........................................
....................................,@#@@@@@#+'';;;;;+@#```````````````````````````##########@........................................
.....................................@``````````.,,,.``````````````````````````````############.......................................
.....................................@`````````````````````````````````````````````#######+'+#@.......................................
.....................................@`````````````````````````````````````````````##########'@.......................................
.....................................#`````````````````````````````````````````````############@#.....................................
.....................................:.````````````````````````````````````````````##############@,...................................
......................................+```````````````````````````````````````````.###############@#..................................
......................................@```````````````````````````````````````````.################@@.................................
......................................@```````````````````````````````````````````.###+##############@................................
......................................@```````````````````````````````````````````.###+###############@...............................
......................................',``````````````````````````````````````````.####'##############@@..............................
.......................................@```````````````````````````````````````````#####+##############@:.............................
.......................................@```````````````````````````````````````````#####'###############@.............................
.......................................@```````````````````````````````````````````######'################............................
.......................................#,``````````````````````````````````````````#######'##############@............................
........................................@``````````````````````````````````````````@######++##############+...........................
........................................@``````````````````````````````````````````@#######'##############@...........................
........................................@``````````````````````````````````````````@########'#############@...........................
.......................................@#'`````````````````````````````````````````@#########'##############..........................
.......................................@#@`````````````````````````````````````````+#########+'############@..........................
......................................@##@`````````````````````````````````````````.##########+'###########@..........................
......................................@##@:`````````````````````````````````````````###########+'###########..........................
.....................................:@###@`````````````````````````````````````````@###########+'+#########,.........................
.....................................@####@`````````````````````````````````````````@#############''########..........................
.....................................@####@.````````````````````````````````````````;##############+'######@..........................
.....................................@#####@`````````````````````````````````````````################@@@###+..........................
.....................................@#####@`````````````````````````````````````````@###############@..;;............................
....................................,@#####@.````````````````````````````````````````+################'...............................
....................................:#######@`````````````````````````````````````````################@...............................
....................................:#######@`````````````````````````````````````````@###############@...............................
....................................,@#######,````````````````````````````````````````:###############@...............................
.....................................@######@@`````````````````````````````````````````@##############@...............................
.....................................@######@@`````````````````````````````````````````+##############@...............................
.....................................@#####@,;;`````````````````````````````````````````@#############@...............................
.....................................@####@@..@`````````````````````````````````````````+#############@...............................
.....................................,####@...@``````````````````````````````````````````@############+...............................
......................................@##@.....@`````````````````````````````````````````:###########@,...............................
.......................................@+......@``````````````````````````````````````````@##########@................................
...............................................:#``````````````````````````````````````````##########@................................
................................................@``````````````````````````````````````````+########@,................................
................................................'+``````````````````````````````````````````@#######@.................................
.................................................@```````````````````````````````````````````@#####@:.................................
.................................................'#``````````````````````````````````````````.#####@..................................
..................................................@```````````````````````````````````````````;###@...................................
...................................................@```````````````````````````````````````````+#@'...................................
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.......................................................;@@'```````````````````````````````:@@+;.......................................
.......................................................@,,'@@'``````````````````````````@@@,,,@.......................................
......................................................@,,,,,,'@@@@;````````````````.+@@@;,,,,,@.......................................
......................................................#@+@,,,,,,,,+@@@@@@@@@@@@@@@@@;,,,,,'@@@........................................
.........................................................+,,,#',,@@..............@,,,,,,,,@...........................................
..........................................................@@@,#@@,...............:+,,,'@,,@...........................................
..................................................................................@,,,@.##............................................
...................................................................................@;@................................................
....................................................................................:.................................................
......................................................................................................................................
......................................................................................................................................
 */
import java.io.IOException;
import java.io.InputStream;
import java.math.BigInteger;
import java.util.*;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import static java.lang.Math.max;
import static java.lang.Math.min;
import static java.lang.Math.pow;
import static java.lang.Math.abs;

import static java.lang.String.format;


public class Main {
    final static int INF = Integer.MAX_VALUE>>1;
    final static int MOD = 1_000_000_007;
    final static int[] dx4 = { 0, 1, 0, -1 };
    final static int[] dy4 = { 1, 0, -1, 0 };
    final static int[] dx8 = {0, 1, 1,  1,  0, -1, -1, -1};
    final static int[] dy8 = {1, 1, 0, -1, -1, -1, 0, 1};
    public static void main(String[] args) {
        Scanner sc=new Scanner();
        int n=sc.nextInt();
        long[][] dp=new long[n+1][4*4*4];
        Arrays.fill(dp[3],1);
        for (int x = 0; x < 4 * 4 * 4; x++) {
            int keta0=(x>>4)&3;
            int keta1=(x>>2)&3;
            int keta2=(x>>0)&3;
            if(Moji.values()[keta0]==Moji.A&&Moji.values()[keta1]==Moji.G&&Moji.values()[keta2]==Moji.C)dp[3][x]=0;
            if(Moji.values()[keta0]==Moji.G&&Moji.values()[keta1]==Moji.A&&Moji.values()[keta2]==Moji.C)dp[3][x]=0;
            if(Moji.values()[keta0]==Moji.A&&Moji.values()[keta1]==Moji.C&&Moji.values()[keta2]==Moji.G)dp[3][x]=0;
        }
        for (int i = 4; i <= n; i++) {
            for (int x = 0; x < 4 * 4 * 4; x++) {
                int keta0=(x>>4)&3;
                int keta1=(x>>2)&3;
                int keta2=(x>>0)&3;
                for (Moji m : Moji.values()) {
                    if(Moji.values()[keta1]==Moji.A&&Moji.values()[keta2]==Moji.G&&m==Moji.C)continue;
                    if(Moji.values()[keta0]==Moji.A&&Moji.values()[keta2]==Moji.G&&m==Moji.C)continue;
                    if(Moji.values()[keta1]==Moji.G&&Moji.values()[keta2]==Moji.A&&m==Moji.C)continue;
                    if(Moji.values()[keta1]==Moji.A&&Moji.values()[keta2]==Moji.C&&m==Moji.G)continue;
                    if(Moji.values()[keta0]==Moji.A&&Moji.values()[keta1]==Moji.G&&m==Moji.C)continue;
                    dp[i][(keta1<<4)+(keta2<<2)+m.ordinal()]+=dp[i-1][x];
                    dp[i][(keta1<<4)+(keta2<<2)+m.ordinal()]%=MOD;
                }
            }
        }
        long ans=0;
        for(long anss:dp[n]){
            ans+=anss;
            ans%=MOD;
        }
        put(ans);

    }
    public static enum Moji{
        A,C,G,T;
    }
    public static class Permutation {
        public static int startIndexOfDescendingOrder(int[] p){
            //return i
            //mean p[i-1]<p[i]&&p[i]>p[i+1]>p[i+2]>...>p[p.length]
            for(int i=p.length-2;i>=0;i--){
                if(p[i]-p[i+1]>=0)continue;
                return i+1;
            }
            return 0;
        }
        public static <T> int startIndexOfDescendingOrder(T[] p,Comparator<? super T> comparator){
            //return i
            //mean p[i-1]<p[i]&&p[i]>p[i+1]>p[i+2]>...>p[p.length]
            for(int i=p.length-2;i>=0;i--){
                if(comparator.compare(p[i],p[i+1])>=0)continue;
                return i+1;
            }
            return 0;
        }
        public static <T> int startIndexOfDescendingOrder(List<T> list,Comparator<? super T> comparator){
            //return i
            //mean p[i-1]<p[i]&&p[i]>p[i+1]>p[i+2]>...>p[p.length]
            for(int i=list.size()-2;i>=0;i--){
                if(comparator.compare(list.get(i),list.get(i+1))>=0)continue;
                return i+1;
            }
            return 0;
        }

        public static boolean nextPermutation(int[] p) {
            int firstIndex=startIndexOfDescendingOrder(p);
            if(firstIndex==0)return false;
            int ng=p.length;
            int ok=firstIndex;
            firstIndex--;
            while(Math.abs(ng-ok)>1){
                int mid=(ng+ok)/2;
                if(p[mid]>p[firstIndex]){
                    ok=mid;
                }else{
                    ng=mid;
                }
            }
            int t = p[firstIndex];
            p[firstIndex] = p[ok];
            p[ok] = t;
            //p[firstIndex+1]以降のものを逆にする
            firstIndex++;
            Arrays.sort(p,firstIndex,p.length);
            return true;
        }
        public static <T> boolean nextPermutation(T[] p,Comparator<? super T> comparator){
            int firstIndex=startIndexOfDescendingOrder(p,comparator);
            if(firstIndex==0)return false;
            int ng=p.length;
            int ok=firstIndex;
            firstIndex--;
            while(Math.abs(ng-ok)>1){
                int mid=(ng+ok)/2;
                if(comparator.compare(p[mid],p[firstIndex])>0){
                    ok=mid;
                }else{
                    ng=mid;
                }
            }
            T t = p[firstIndex];
            p[firstIndex] = p[ok];
            p[ok] = t;
            //p[firstIndex+1]以降のものを逆にする
            //現在降順で昇順に並べる
            //ソートのほうが簡潔に書ける
            firstIndex++;
            Arrays.sort(p,firstIndex,p.length);
            return true;
        }
        public static <T> boolean nextPermutation(List<T> list,Comparator<T> comparator) {
            int firstIndex=startIndexOfDescendingOrder(list,comparator);
            if(firstIndex==0)return false;
            int ng=list.size();
            int ok=firstIndex;
            firstIndex--;
            //MGR式
            while(Math.abs(ng-ok)>1){
                int mid=(ng+ok)/2;
                if(comparator.compare(list.get(mid),list.get(firstIndex))>0){
                    //真に大きいならokの領域
                    ok=mid;
                }else{
                    ng=mid;
                }
            }
            T t = list.get(firstIndex);
            list.set(firstIndex,list.get(ok));
            list.set(ok,t);
            //インデックスがfirstIndex+1以降のものを逆にする
            firstIndex++;
            Collections.reverse(list.subList(firstIndex,list.size()));
            return true;
        }
        public static <T> boolean prevPermutation(List<T> list,Comparator<T> comparator){
            return nextPermutation(list,comparator.reversed());
        }
    }


    public static void print(Object object){
        System.out.print(object);
    }
    public static void put(Object object) {
        System.out.println(object);
    }
    public static void put(){
        System.out.println();
    }

    public static void printE(Object object){
        System.err.print("DEBUG:"+object);
    }
    public static void putE(Object object){
        System.err.println("DEBUG:"+object);
    }
    public static void putE(){
        System.err.println("DEBUG:");
    }

    public static void print(String format,Object... args){
        System.out.print(String.format(format,args));
    }
    public static void put(String format,Object... args) {
        System.out.println(String.format(format,args));
    }
    public static void printE(String format,Object... args){
        System.err.print("DEBUG:"+String.format(format,args));
    }
    public static void putE(String format,Object... args){
        System.err.println("DEBUG:"+String.format(format,args));
    }

    final static private class Scanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;

        private boolean hasNextByte() {
            if (ptr < buflen) {
                return true;
            } else {
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0) {
                    return false;
                }
            }
            return true;
        }

        private int readByte() {
            if (hasNextByte())
                return buffer[ptr++];
            else
                return -1;
        }

        private boolean isPrintableChar(int c) {
            return 33 <= c && c <= 126;
        }

        public boolean hasNext() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr]))
                ptr++;
            return hasNextByte();
        }

        public String next() {
            if (!hasNext())
                throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        public long nextLong() {
            if (!hasNext())
                throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }

        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
                throw new NumberFormatException();
            return (int) nl;
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }
    }
    final static private class FixedIntPair {
        final public int x, y;
        final static public FixedIntPair ZEROS=new FixedIntPair(0,0);
        FixedIntPair(int x, int y) {
            this.x = x;
            this.y = y;
        }
        public static double distance(FixedIntPair fip1,FixedIntPair fip2){
            double x = (double) fip1.x - fip2.x;
            double y = (double) fip1.y - fip2.y;
            return Math.sqrt(x*x+y*y);
        }

        @Override
        public int hashCode() {
            return x+y;
        }

        @Override
        public boolean equals(Object obj) {
            boolean result=super.equals(obj);
            if(obj.getClass()!=this.getClass()){
                return false;
            }
            FixedIntPair pair=(FixedIntPair) obj;
            if(this.x==pair.x&&this.y==pair.y) return true;
            return false;
        }

        @Override
        public String toString() {
            return String.format(FixedIntPair.class.getSimpleName()+":(%d,%d)", x, y);
        }
    }
    final static private class FixedLongPair {
        final public long x, y;
        final static public FixedLongPair ZEROS=new FixedLongPair(0,0);
        FixedLongPair(long x, long y) {
            this.x = x;
            this.y = y;
        }
        public static double distance(FixedLongPair flp1,FixedLongPair flp2){
            double x = (double) flp1.x - flp2.x;
            double y = (double) flp1.y - flp2.y;
            return Math.sqrt(x*x+y*y);
        }

        @Override
        public int hashCode() {
            return (int)x+(int)y;
        }

        @Override
        public boolean equals(Object obj) {
            boolean result=super.equals(obj);
            if(obj.getClass()!=this.getClass()){
                return false;
            }
            FixedLongPair pair=(FixedLongPair)obj;
            if(this.x==pair.x&&this.y==pair.y) return true;
            return false;
        }

        @Override
        public String toString() {
            return String.format(FixedLongPair.class.getSimpleName()+":(%d,%d)", x, y);
        }
    }
    final static private class Binary{
        public static String toZeroPadding(int i){
            return String.format("%"+Integer.toBinaryString(-1).length()+"s",Integer.toBinaryString(i)).replace(' ','0');
        }
        public static String toZeroPadding(long i){
            return String.format("%"+Long.toBinaryString(-1).length()+"s",Long.toBinaryString(i)).replace(' ','0');
        }
    }

    final static private class Util {
        static long gcd(long a,long b){
            //最大公約数　
            if(a%b==0)return b;
            return gcd(b,a%b);
        }
        static long lcm(long a,long b){
            //最小公倍数
            long gcd=gcd(a,b);
            long result=b/gcd;
            return a*result;
        }
        static <T extends Comparable<T>> Map<T,Integer> count(List<T> list){
            //副作用
            Collections.sort(list);
            Map<T,Integer> result=new HashMap<>();
            int l=0,r=0;
            while(l<list.size()){
                while(r<list.size()-1&&list.get(r).equals(r+1)){
                    r++;
                }
                result.put(list.get(r),r-l+1);
                r++;
                l=r;
            }
            return result;

        }
        static Map<Integer,Integer> count(int[] array){
            //副作用
            Arrays.sort(array);
            Map<Integer,Integer> result=new HashMap<>();
            int l=0,r=0;
            while(l<array.length){
                while(r<array.length-1&&array[r]==array[r+1]){
                    r++;
                }
                result.put(array[l],r-l+1);
                r++;
                l=r;
            }
            return result;
        }
        static <T> String toStringBWS(Iterable<T> iterable){
            Iterator<T> ite=iterable.iterator();
            return toStringBWS(ite);
        }
        static <T> String toStringBWS(Iterator<T> ite){
            StringBuilder sb=new StringBuilder();
            sb.append(ite.next());
            while(ite.hasNext()){
                sb.append(" ");
                sb.append(ite.next());
            }
            return sb.toString();
        }
        static long factoringInPrimeNumbers(long n,int[] a){
            //素因数分解
            for(int i=2;n>1&&i< a.length;i++){
                while(n%i==0){
                    a[i]++;
                    n/=i;
                }
            }
            return n;

        }
        static boolean isValidCell(int i,int j,int h,int w){
            return i>=0&&i<h&&j>=0&&j<w;
        }
    }
}