#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;
}
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