char edge[100][100];
int main()
{
    //seive();
    //freopen("Nayem.txt", "r", stdin);
    Fast_read

    ll n;
    cin>>n;

    for (int i=0;i<n;i++)
    {
        for (int j=0;j<n;j++)
        {
            cin>>edge[i][j];
        }
    }

    for (int i=1;i<n-1;i++)
    {
        for (int j=1;j<n-1;j++)
        {
            if (edge[i][j]=='.' && edge[i-1][j]=='.' && edge[i+1][j]=='.' && edge[i][j-1]=='.' && edge[i][j+1]=='.')
            {
                edge[i][j]='#';
                edge[i+1][j]='#';
                edge[i-1][j]='#';
                edge[i][j+1]='#';
                edge[i][j-1]='#';
            }
        }
    }

    for (int i=0;i<n;i++)
    {
        for (int j=0;j<n;j++)
        {
            if (edge[i][j]=='.')
            {
                cout<<"NO"<<endl;
                return 0;
            }
        }
    }

    cout<<"YES"<<endl;


    return 0;
}