第四题：
import java.util.Scanner;
public class Problem4 {
private static void solution(int k) {
long[][] recorder = new long[4][k + 1];
recorder[0][0] = 1;
long mod = 1000000007;
for (int i = 1; i < k + 1; ++ i) {
for (int j = 0; j < 4; ++ j) {
for (int m = 0; m < 4; ++ m) {
if (m == j) {
continue;
}
recorder[j][i] += recorder[m][i - 1] % mod;
recorder[j][i] %= mod;
}
recorder[j][i] %= mod;
}
}
System.out.println(recorder[0][k] % mod);
}
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int k = scanner.nextInt();
solution(k);
}
}
