#include <iostream>
using namespace std;

const int mod = 1000000007;

int main() {
    int n;
    cin >> n;
    int x;
    int elozo=-1;
    int ans = 1;
    for(int i = 0; i < n; i++)
        {
        cin >> x;
        if(x == elozo)
            {
            ans = (ans+ans-1)%mod;
            }
        else
            {
            ans = (ans * 2) % mod;
            }
        }
    cout << ans;
    return 0;
}