#include <bits/stdc++.h>
using namespace std;
int t, n, m, k, l, r, op, x, y;
string tmp, str;
int siz = 26;
void solve() {
    for (int i = 0; i < siz; i++) {
        tmp.push_back('a' + i);
    }
    while (cin >> str) {
        x = y = 0;
        stack<char> stk;
        while (true) {
            while (!stk.empty() && stk.top() == str[y]) {
                stk.pop();
                y++;
            }
            if (x == siz)break;
            stk.push(tmp[x]);
            x++;
        }
        cout << (stk.empty() ? "yes" : "no") << "\n";
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    solve();
    return 0;
}