#include <bits/stdc++.h>
using namespace std;

int main()
{
    int n;
    cin >> n;
    int n1 = 0;
    int n12 = 0;
    int ans = 0;
    int mod = 1000000007;
    for(int c1 = 0; c1 < n; c1++){
        int a;
        cin >> a;
        if(a == 1){
            n1++;
        }
        if(a == 2){
            n12 = 2*n12 + n1;
            n12 %= mod;
        }
        if(a == 3){
            ans += n12;
            ans %= mod;
        }
    }
    cout << ans << "\n";
    return 0;
}