C :

#include <stdio.h>

int main(){
	
	int N, i;
	long long int a[64];
	
	a[0] = 0;
	for (i = 1; i < 36; i++){
		a[i] = 3 * a[i-1] + 2;
	}
	
	while (EOF != scanf("%d", &N)){
		
		printf("%lld\n", a[N]);
	}
	
	return 0;
}

C++ :

#include<stdio.h>

long long f(int n)
{
	if(n==1)
		return 2;
	return f(n-1)*3+2;
}

int main()
{
	int n;
	while(scanf("%d",&n)!=EOF)
		printf("%lld\n",f(n));
	return 0;
}