#include<bits/stdc++.h>
#define TC while(t--)
#define DEBUG(x) cout<<"Debug "<<#x<<':'<<x<<endl;
#define EL "\n"
#define ll long long
#define pii pair<int,int>
#define sz(x) (int)x.size()
#define st first
#define nd second
#define pb push_back
using namespace std;
void fast(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
}
const int MOD = 1e9+7;
void solve(){
	int n;
	cin>>n;
	vector<ll> arr(n+1),qs_1(n+1),qs_2(n+2);
	qs_1[0]=qs_2[0]=0;
	for(int i=1;i<=n;i++){
		cin>>arr[i];
		qs_1[i]=((arr[i]==1)?qs_1[i-1]+1:qs_1[i-1]);
		qs_1[i]%=MOD;
		qs_2[i]=((arr[i]==2)?qs_2[i-1]*2+qs_1[i-1]:qs_2[i-1]);
		qs_2[i]%=MOD;
	}
	ll sum=0;
	for(int i=1;i<=n;i++){
		if(arr[i]==3){
			sum+=qs_2[i];
			sum%=MOD;
		}
	}
	cout<<sum;
}
int main(){
	fast();
	solve();
	/*int t;
	cin>>t;
	TC{
		solve();
	}*/
	return 0;
}