Valid Username
Usernames are 3–16 characters of lowercase letters, digits or underscores, and must start with a lowercase letter. Is S valid?
Input: one line with the candidate string S.
Output: YES or NO.
Constraints:
- 1 ≤ |S| ≤ 100
Sample Tests
Input 1
coder_99
Output 1
YES
Input 2
9lives
Output 2
NO
Discussion (opens after you solve — no spoilers before)
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