#pragma GCC optimize("O3,unroll-loops")
#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define Waimai ios::sync_with_stdio(false), cin.tie(0)
#define FOR(x,a,b) for(int x = a, I = b; x <= I; x++)
#define pb emplace_back
#define F first
#define S second

const int MOD = 1e9 + 7;
const int SIZE = 1e6 + 5;

int n, ans;
int a[SIZE], p2[SIZE], i2[SIZE];

void solve() {
    cin >> n;
    FOR (i, 1, n) cin >> a[i];
    p2[0] = i2[0] = 1;
    i2[1] = (MOD + 1) / 2;
    FOR (i, 1, n) {
        p2[i] = 2 * p2[i - 1] % MOD;
        i2[i] = 1ll * i2[i - 1] * i2[1] % MOD;
    }
    int sum = 0, c2 = 0, c1 = 0;
    FOR (i, 1, n) {
        if (a[i] == 1) {
            sum += i2[c2];
            sum %= MOD;
            c1++;
        }
        if (a[i] == 2) c2++;
        if (a[i] == 3) {
            ans = (ans + 1ll * sum * p2[c2]) % MOD;
            ans -= c1;
            if (ans < 0) ans += MOD;
        }
    }
    cout << ans << '\n';
}

int main() {
    Waimai;
    solve();
}