#include <iostream>

using namespace std;

int main()
{
    char arr[] = "ABCD";

    for (int i = 0; i < 4; i++)
        for (int j = i; j < 4; j++)
            for (int k = j; k < 4; k++)
                for (int v = k; v < 4; v++)
                    cout << arr[i] << arr[j] << arr[k] << arr[v] << endl;


    return 0;
}