N queens

using System;

class Program{
    public static void Main(){
        int n = int.Parse(Console.ReadLine());
        if((n == 2) || (n == 3)){
            Console.WriteLine("Impossible");
            Console.ReadKey();
            return ;
        }
        char[ , ] a = new char [n , n];
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                a[i , j] = '.';
            }
        }
        bool ok = false;
        int y = 0;
        bool[] used = new bool[40* n + 2];
        int[] left = new int[40 * n];
        int[] right = new int[40 * n];
        for(int i = 0; i < n; i++){
            solve(i , y , n , ref ok , ref left , ref right , ref used, ref a);
            if(ok)
                break;
        }
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                Console.Write(a[i , j]);
            }
            Console.Write('\n');
        }
        Console.ReadKey();
    }
    public static void solve(int x , int y , int n, ref bool ok , ref int[] left , ref int[] right , ref bool[] used , ref char[,] a){
        if(ok == true){
            return ;
        }
        if(y == (n - 1)){
            a[x , y] = 'Q';
            ok = true;
            return ;
        }
        a[x , y] = 'Q';
        used[x] = true;
        left[3 * n - x  - y]++;
        right[3 * n + x - y]++;
        for(int i = 0; i < n; i++){
            if(used[i] == false){
                if((left[3 * n - i - (y + 1)] == 0) && (right[3 * n + i - (y + 1)] == 0)){
                    solve(i , (y + 1) , n , ref ok , ref left , ref right , ref used , ref a);
                    if(ok == true){
                        return ;
                    }
                }
            }
        }
        used[x] = false;
        left[3 * n - x  - y]--;
        right[3 * n + x - y]--;
        a[x , y] = '.';
    }
}