#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007;
// const int MOD = 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
const int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  string s; getline(cin, s);
  int n = s.length();
  REP(i, n) FOR(j, i + 1, n) {
    string t = s;
    vector<bool> era(26, false);
    REP(k, i) {
      if (t[k] != ' ') era[t[k] - 'a'] = true;
    }
    for (int k = i + 1; k <= j; k += 2) {
      if (t[k] != ' ') era[t[k] - 'a'] = true;
    }
    FOR(k, j + 1, n) {
      if (t[k] != ' ') era[t[k] - 'a'] = true;
    }
    for (char &c : t) {
      if (c != ' ' && era[c - 'a']) c = ' ';
    }
    bool ok = true;
    for (int k = i; k <= j; k += 2) {
      if ('a' > t[k] || t[k] > 'z') {
        ok = false;
        break;
      }
      if (k + 1 <= j && t[j + 1] != ' ') {
        ok = false;
        break;
      }
    }
    if (ok) {
      cout << "Yes\n";
      return 0;
    }
  }
  cout << "NO\n";
  return 0;
}