#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;

#define REP(i,s,e) for (i = s; i <= e; i++)
#define rep(i,n) REP (i,0,(int)(n)-1)
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP (i,(int)(n)-1,0)
#define INF (int)1e8
#define MOD (int)(1e9+7)

typedef long long ll;

int cnt[11][256];

int main(void) {
    int i, j, n;
    set<string> st;
    string s;
    cin >> n;
    while (n--) {
        cin >> s;
        st.insert(s);
    }
    s = "aabegimnruu";
    do {
        if (!st.count(s)) {
            cout << s << endl;
            return 0;
        }
    } while (next_permutation(s.begin(),s.end()));
    cout << "NO" << endl;
    return 0;
}