View Code of Problem 3692

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