文件目录

#include <iostream>
using namespace std;
int main()
{
    string s;
    cin >> s;
    if (s[0] >= 48 && s[0] <= 57)
        cout << "no";
    else
    {
        for (int i = 0; s[i]; i++)
        {
            if (s[i] >= 48 && s[i] <= 57)
                continue;
            if (s[i] >= 65 && s[i] <= 90)
                continue;
            if (s[i] >= 97 && s[i] <= 122)
                continue;
            if (s[i] == '_')
                continue;
            cout << "no";
            return 0;
        }
        cout << "yes";
    }
    return 0;
}