#include <bits/stdc++.h> using namespace std; int main(){ string n; cin>>n; if(n[0]>='0'&&n[0]<='9'){ cout<<"no"; return 0; } for(int i=0;i<n.length();i++){ if((n[i]>='0'&&n[i]<='9')||(n[i]>='A'&&n[i]<='Z')||(n[i]>='a'&&n[i]<='z')||n[i]==''){ continue; }else{ cout<<"no"; return 0; } } cout<<"yes"; return 0; } //@@dingsheng 感谢你的代码！！~~

#include <bits/stdc++.h>
using namespace std;
int main() {
    string s;
    cin >> s;
    int n = s.length();
    if (n == 0 || n > 20) {
        cout<<"no";
        return 0;
    }
    if (isdigit(s[0])) {
        cout<<"no";
        return 0;
    }
    for (int i = 0; i < n; ++i) {
        if (!isalnum(s[i]) && s[i] != '_') {
            cout<<"no";
            return 0;
        }
    }
    cout<<"yes";
    return 0;
}
\\沙威玛，沙威玛，沙威玛 🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯🌯