#include <stdio.h>


int n, d[1010];
long count;


void read()
{
int i;
scanf("%d\n", &n);


for(i = n; i > 0; --i)
	scanf("%d ", &d[i]);
}


void h(int x, int i, int j)
{

if(x > 1)
	{



		h(x-1, i, 6-i-j);
		count += d[x];


		//printf("%d -> %d\n", i, j);



		h(x-1, 6-i-j, j);
	}
else
	{
		//printf("%d -> %d\n", i, j)
		count += d[x];
	}
}



int main()
{
freopen("hanoig.in", "r", stdin);
freopen("hanoig.out", "w", stdout);


read();



h(n, 1, 2);

printf("%d\n", count);


fclose(stdin);
fclose(stdout);


return 0;
}