poj1205

递推

import java.io.*;
import java.util.*;
import java.math.*;

public class Main {

    public static void main(String args[])
    {
        Scanner cin = new Scanner(new BufferedInputStream(System.in));
        BigInteger[][] f = new BigInteger[101][3];
        f[0][0] = new BigInteger("0");
        f[0][1] = new BigInteger("1");
        f[0][2] = new BigInteger("0");
        BigInteger[] res = new BigInteger[101];
        for (int i = 1; i <= 100; i++)
        {
            f[i][0] = f[i - 1][0].add(f[i - 1][2]);
            f[i][1] = f[i - 1][1].add(f[i - 1][2]).add(f[i - 1][0]);
            f[i][2] = f[i - 1][1].add(f[i - 1][2]).add(f[i - 1][0]);
            res[i] = f[i][0].add(f[i][2]);
        }
        while (cin.hasNext())
        {
            int a = cin.nextInt();
            System.out.println(res[a]);
        }
    }
}