C :

#include<stdio.h>
int main()
{
	int n,i,a[31];
	a[0]=1;
	a[2]=3;
	a[4]=11;
	for(i=6;i<31;i+=2)
		a[i]=a[i-2]*4-a[i-4];
	while(scanf("%d",&n)!=EOF,n!=-1)
	{
		if(n%2==0)
			printf("%d\n",a[n]);
		else
			printf("0\n");
	}
	return 0;
}