n = int(input())
lis = list(map(int,input().split()))
num = 0
ans = 0
for i in range(n//2):
    num += 2 ** i
    ans += lis[i] * num
    ans += lis[-(1 + i)] * num
if len(lis)%2:ans -= lis[(n-1)//2]
print(ans)