poj2680

递推+高精度

java做

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

public class Main {
    public static void main(String[] args)
    {
        Scanner cin = new Scanner(new BufferedInputStream(System.in));
        BigInteger f[] = new BigInteger[1005];
        f[0] = new BigInteger("0");
        f[1] = new BigInteger("1");
        f[2] = new BigInteger("1");
        for (int i = 3; i <= 1001; i++)
            f[i] = f[i - 2].add(new BigInteger("2").pow(i - 2));
        while (cin.hasNext())
        {
            int n = cin.nextInt();
            System.out.println(f[n - 1]);
        }
    }
}