n = int(input())
seql = list(map(int, input().split()))
import operator, itertools

n -= 2
ans = 0
for p in itertools.accumulate(seql, func=operator.mul):
    if not p:
        break
    ans += pow(3, n, 10**9+7) * 2*p if n >= 0 else p
    n -= 1
print (ans%(10**9+7))