#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))

using namespace std;
typedef pair<llint, llint> P;

string s;
bool used[26];

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	getline(cin, s);
	llint n = s.size();
	
	for(int i = 0; i < n; i++){
		for(int j = 0; j < n; j++){
			if(i > j) continue;
			
			bool ans = true;
			for(int k = 0; k < 26; k++) used[k] = false;
			for(int k = i; k <= j; k+=2){
				if(s[k] == ' '){
					ans = false;
					break;
				}
				else used[s[k]-'a'] = true;
			}
			
			for(int k = 0; k < i; k++){
				if(s[k] != ' ' && used[s[k]-'a']) ans = false;
			}
			for(int k = j+1; k < n; k++){
				if(s[k] != ' ' && used[s[k]-'a']) ans = false;
			}
			for(int k = i+1; k <= j; k+=2){
				if(s[k] != ' ' && used[s[k]-'a']) ans = false;
			}
			if(ans){
				cout << "Yes" << endl;
				return 0;
			}
		}
	}
	cout << "NO" << endl;
	
	return 0;
}