UVA - 580 - Critical Mass (Java solution)

/*by trial i found that  the pattern that array follows is f[n]=2*f[n-1]+sumof[last 3 remainders]
 ex:
 f[i] =2 *f[i-1] + f[i-1]%f[i-2] + f[i-2]%f[i-3] + f[i-3]%f[i-4]
as i>7
or  f[i] =2 *f[i-1]+add[i]
as add works for all values of i>2
*/

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws IOException {
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        StringBuffer sb=new StringBuffer("");
       
        long[] add=new long[31];
        add[3]=1;
        add[4]=2;
        add[5]=4;
        for(int i=6;i<31;i++){
            add[i]=add[i-3]+add[i-2]+add[i-1];
           
        }
       
        long []bg=new long[31];
        bg[0]=0;
        bg[1]=0;
        bg[2]=1;
        for(int i=3;i<31;i++){
            bg[i]=2*bg[i-1]+add[i];
        }
        while(true){
            int x=Integer.parseInt(br.readLine());
            if(x==0)
                break;
            sb.append(bg[x-1]).append("\n");
        }
        System.out.println(sb);
    }
}