from operator import mul
from functools import reduce
def cmb(n,r):
    r=min(n-r,r)
    if r==0: return 1
    over=reduce(mul,range(n,n-r,-1))
    under=reduce(mul,range(1,r+1))
    return over//under

n=int(input())
a=list(map(int,input().split()))
ans=0
for i in range(n):
    ans+=a[i]*cmb(n-1,i)
print(ans)