#include<stdio.h>

int a[2001][330];

int main()
{
    int n;

    for(int i=0; i<2001; i++)
    {
        for(int j=0; j<330; j++)
        {
            a[i][j]=0;
        }
    }

    scanf("%d", &n);

    a[1][329]=1;
    a[2][329]=4;
    a[3][329]=8;

    for(int i=4; i<2001; i++)
    {
        for(int j=329; j>0; j--)
        {
            if((i%2)==0)
            {
                a[i][j]=a[i][j]+(2*a[i-1][j])%10-a[i-2][j];
                if(a[i][j]<0)
                {
                    a[i][j]=a[i][j]+10;
                    a[i][j-1]=a[i][j-1]-1;
                }
                if(a[i][j]>10)
                {
                    a[i][j]=a[i][j]%10;
                    a[i][j-1]=a[i][j-1]+a[i][j]/10;
                }
                if(2*a[i-1][j]>9)
                {
                    a[i][j-1]=a[i][j-1]+(2*a[i-1][j])/10;
                }
            }
            else
            {
                a[i][j]=a[i][j]+(3*a[i-1][j])%10-(2*a[i-2][j])%10;
                if(a[i][j]<0)
                {
                    a[i][j]=a[i][j]+10;
                    a[i][j-1]=a[i][j-1]-1;
                }
                if(a[i][j]>10)
                {
                    a[i][j]=a[i][j]%10;
                    a[i][j-1]=aa[i][j-1]+[i][j]/10;
                }
                if(3*a[i-1][j]>9)
                {
                    a[i][j-1]=a[i][j-1]+(3*a[i-1][j])/10;
                }
                if(2*a[i-2][j]>9)
                {
                    a[i][j-1]=a[i][j-1]-(2*a[i-2][j])/10;
                }
            }
        }
    }

    for(int i=0; i<330; i++)
    {
        if(a[n][i]!=0)
        {
            for(int j=i; j<330; j++)
            {
                printf("%d", a[n][j]);
            }
            printf("\n");
            goto A;
        }
    }
A:
    return 0;
}

All tests are right, but I can't understand why WA12?