PROGRAMMING:Judge legal identifier
Write a program to determine the legal identifier. It is required to input a positive integer repeat (0 < repeat < 10) and perform the following operations:
Enter a character string ending with carriage return (less than 80 characters) to determine whether it conforms to the identifier specified by C + + (a string composed of letters, numbers and underscores starting with letters or underscores).
Input and output example: description in brackets
###Input example:
```in
4 (repeat=4)
1num
_ abc
Int_ two
Abc def
```
###Output example:
```out
No
Yes
Yes
No
```
answer：If there is no answer, please comment
Enter a character string ending with carriage return (less than 80 characters) to determine whether it conforms to the identifier specified by C + + (a string composed of letters, numbers and underscores starting with letters or underscores).
Input and output example: description in brackets
###Input example:
```in
4 (repeat=4)
1num
_ abc
Int_ two
Abc def
```
###Output example:
```out
No
Yes
Yes
No
```
answer：If there is no answer, please comment