BZOJ 1002 轮状病毒(生成树个数)

import java.util.*;
import java.text.*;
import java.math.*;


public class Main{
 
    public static void PR(String s){
        System.out.println(s);
    }
    
    public static void PR(int x)
    {
        System.out.println(x);
    }
    
      public static void PR(BigInteger x)
    {
        System.out.println(x);
    }
    
    public static void PR(double s)
    {
        java.text.DecimalFormat d=new java.text.DecimalFormat("#.000000");
        System.out.println(d.format(s));
    }
    
    
    static BigInteger a,b,c;
    static int n;
    
    
   
    public static void main(String[] args){
        
        Scanner S=new Scanner(System.in);
        while(S.hasNext())
        {
            n=S.nextInt();
            a=BigInteger.ONE;
            b=BigInteger.valueOf(5);
            int i;
            if(n==1) PR(a);
            else if(n==2) PR(b);
            else
            {
                for(i=3;i<=n;i++) 
                {
                    c=b.multiply(BigInteger.valueOf(3));
                    c=c.subtract(a).add(BigInteger.valueOf(2));
                    a=b;
                    b=c;
                }
                PR(c);
            }
        }
    }
}