PROGRAMMING:The problem of alphabet combination
How many combinations of $$n (0 < n / Leq m) $$letters can be selected from the $$m (0 < m / Leq 26) $$capital letters?
Please write a program, input $$M $$and $$n $$and output any combination of $$n $$letters in the continuous $$M $$letters starting from a.
Requirements: output a combination for each line (the letters in the combination are sorted in ascending order), and output in dictionary order.
####Input sample
```in
4 3
```
####Output sample
```out
ABC
ABD
ACD
BCD
```
answer：If there is no answer, please comment
Please write a program, input $$M $$and $$n $$and output any combination of $$n $$letters in the continuous $$M $$letters starting from a.
Requirements: output a combination for each line (the letters in the combination are sorted in ascending order), and output in dictionary order.
####Input sample
```in
4 3
```
####Output sample
```out
ABC
ABD
ACD
BCD
```
answer：If there is no answer, please comment