#!/usr/bin/env python3
n = int(input())
a = list(map(int, input().split()))
r = 0
for i in range(len(a)):
    for j in range(i):
        if a[i] != 3 or a[j] != 1: continue
        x = 1
        for k in range(j, i):
            if a[k] == 2: x = x*2
        r += x - 1
print(r%1000000007)