Problem:
Score : 100 points

Problem Statement

Decades have passed since the beginning of AtCoder Beginner Contest.

The contests are labeled as ABC001, ABC002, ... from the first round, but after the 999-th round ABC999, a problem occurred: how the future rounds should be labeled?

In the end, the labels for the rounds from the 1000-th to the 1998-th are decided: ABD001, ABD002, ..., ABD999.

You are given an integer N between 1 and 1998 (inclusive). Print the first three characters of the label of the N-th round of AtCoder Beginner Contest.

Constraints

1 \leq N \leq 1998

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the first three characters of the label of the N-th round of AtCoder Beginner Contest.

Sample Input 1

999

Sample Output 1

ABC

The 999-th round of AtCoder Beginner Contest is labeled as ABC999.

Sample Input 2

1000

Sample Output 2

ABD

The 1000-th round of AtCoder Beginner Contest is labeled as ABD001.

Sample Input 3

1481

Sample Output 3

ABD

The 1481-th round of AtCoder Beginner Contest is labeled as ABD482.

Solution in C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

void swap(int * a, int * b)
{
    int t = *a;
    *a = *b;
    *b = t;
}

void print_int(int n)
{
    printf("%d\n",n);
    
}

void sort(int a[], int n)
{
    int h, i, j;

    for(h = 1; h < n / 9; h = h * 3 + 1)
        continue;

    for(; h > 0; h /= 3)
    {
        for(i = h; i < n; i++)
        {
            j = i;
            while(j >= h && a[j] < a[j-h])
            {
                swap(&a[j], &a[j-h]);
                j -= h;
            }
        }
    }
}

void print_list(int a[], int n)
{
    for(int i = 0; i < n; i++)
    {
        printf("%d ", a[i]);
    }
}

void intcpy(int a[], const int b[], int count)
{
    for(int i = 0; i < count; i++)
    {
        a[i] = b[i];
    }
}

int list_sum(int a[], int n)
{
    int sum = 0;

    for(int i = 0; i < n; i++)
    {
        sum += a[i];
    }

    return sum;
}


void permutate(int s[], int count, int n)
{
    if(n == count)
        return;

    for(int i = n; i < count; i++)
    {
        swap(&s[i], &s[n]);

        if(i != n)
        {
            //intcpy(result[j++], s, count);
        }
        
        permutate(s, count, n+1);
        swap(&s[i], &s[n]);
    }
}



int main(void)
{
    int n;
    scanf("%d", &n);
    if(n <= 999)
    {
        printf("%s\n", "ABC");
    }
    else
    {
        printf("%s\n", "ABD");
    }
    
    
    return 0;
}