Tri Tiling

Description 
In how many ways can you tile a 3xn rectangle with 2x1 dominoes? 

Here is a sample tiling of a 3x12 rectangle.

Input 
Input consists of several test cases followed by a line containing -1. Each test case is a line containing an integer 0 <= n <= 30.

Output 
For each test case, output one integer number giving the number of possible tilings.

Sample Input 
2 
8 
12 
-1

Sample Output 
3 
153 
2131 

Source 
Waterloo local 2005.09.24

source code

#include<stdio.h>     long long a[31];    int main()    {        long long n;      int i;        a[0]=1;      a[2]=3;        for(i=4;i<=30;i+=2)            a[i]=4*a[i-2]-a[i-4];        while(scanf("%lld",&n))        {            if(n==-1) break;            printf("%lld\n",a[n]);        }    }