小兔的棋盘

import java.util.*;
 import java.math.*;
 public class Main
 {
     public static void main(String []args)
     {
         Scanner cin=new Scanner(System.in);
         int i,n;
         int count=1;
         while(cin.hasNext())
         {    
             n=cin.nextInt();
             if(n==-1)
             break;
             BigInteger s1 = BigInteger.valueOf(1);
             BigInteger s2 = BigInteger.valueOf(1);
             for(i=1;i<=n;i++)
             {
                 s1=s1.multiply(BigInteger.valueOf(2*n-i+1));
                 s2=s2.multiply(BigInteger.valueOf(i));
             }
             s1=s1.divide(BigInteger.valueOf(n+1));
             s1=s1.divide(s2);
             s1=s1.multiply(BigInteger.valueOf(2));
             System.out.println(count+" "+n+" "+s1);
             count++;    
         }
     }
 }

这个题目自己算了n=4的时候结果是28，发现n=3的时候结果是10,n=1的时候结果是2，就觉得这个题目和

How Many Trees?很相似，这里就是

How Many Trees?里面结果的二倍，发现之后果断用Java写了代码，拿12进行测试，没想到真是！！