#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
using namespace std;
typedef long long ll;
typedef vector<ll> vi;
int cnt[4];
const ll MOD = 1000000007;
int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int n; cin >> n; vi a(n);
    for (auto &x: a) {
        cin >> x;
        if (x == 3) cnt[3]++;
    }
    ll dap = 0, cur = 0;
    for (int i = 0; i < n; i++) {
        if (a[i] == 1) {
            cnt[1]++;
        }
        if (a[i] == 2) {
            (dap += cnt[1] % MOD * cnt[3]) %= MOD;
            (dap += cur * cnt[3]) %= MOD;
            (cur *= 2) %= MOD;
            (cur += cnt[1]) %= MOD;
        }
        if (a[i] == 3) {
            cnt[3]--;
        }
    }
    cout << dap << '\n';
    return 0;
}