#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
void add(int &x, int y)
{
    if ((x += y) >= mod)
        x -= mod;
}
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n;
    cin >> n;
    vector<int> a(n + 1);
    for (int i = 1; i <= n; ++i)
    {
        cin >> a[i];
    }
    int dp1 = 0, dp2 = 0;
    int ans = 0;
    for (int i = 1; i <= n; i++)
    {
        if (a[i] == 1)
        {
            dp1++;
        }
        if (a[i] == 2)
        {
            dp2 = (2ll * dp2 + dp1) % mod;
        }
        if (a[i] == 3)
        {
            add(ans, dp2);
        }
    }
    cout << ans << "\n";
    return 0;
}