C :

#include<stdio.h>
int main()
{
    int n,i;
    double a[51];
    a[1]=3;
	a[2]=6;
	a[3]=6;
    for(i=4;i<=50;i++)
        a[i]=a[i-2]*2+a[i-1];
    while(scanf("%d",&n)!=EOF)
        printf("%.0lf\n",a[n]);
    return 0;
}

C++ :

#include<iostream>
#include<cstdio>
long long a[51]={0};
using namespace std;
int main()
{
   int n,i;
   a[0]=3;a[1]=a[2]=6;
   for(i=3;i<50;i++)
   {
      a[i]=a[i-1]+2*a[i-2];
   }
   while(scanf("%d",&n)!=EOF)
   {
       printf("%lld\n",a[n-1]);
   }
   return 0;
}