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 JavaScript (node.js v5.12) (judge verdict: Wrong Answer)

function StoneMonument(input) {
    input = input.split("\n");
    tmp = input[0].split(" ");
    var a = parseInt(tmp[0], 10);
    var b = parseInt(tmp[1], 10);
    var aa=[0];
    for(var i=1;i<=999;i++){
        aa[i]=i+aa[i-1];
    }
    for(var j=0;j<499500;j++){
        for(var i=j;i<999;i++){
            if(j > b)
                continue;
            if((aa[i-1]-j)==a && (aa[i]-j)==b){
                console.log(j);
                break;
            }
        }
    }
}
StoneMonument(require("fs").readFileSync("/dev/stdin", "utf8"));