#include 
 
using namespace std;
 
int main()
{
    int N;
    cin >> N;
 
    int a[N];
 
    vector v;
 
    for (int i = 0; i < N; i++)
    {
        cin >> a[i];
        if (a[i] == 3)
            v.push_back(i + 1);
    }
 
    v.push_back(N + 1);
 
    int MAX = 0;
    for (int i = 0; i <= v.size() - 5; i++)
        MAX = max(MAX, v[i + 4] - v[i] - 1);
 
    cout << MAX;
 
    return 0;
}