View Code of Problem 3692

#include<stdio.h>
int main(){
	int n,i;
	while(scanf("%d",&n)!=EOF){
		long long int a[50]={0,3,6,6};
		if(n<4){
			for(i=0;i<n;i++);
			printf("%lld\n",a[i]);
		}
		else{
			for(i=4;i<=n;i++)
			a[i]=a[i-1]+a[i-2]*2;
			printf("%lld\n",a[i-1]);
		}
	}
}