import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        inabameguru solver = new inabameguru();
        solver.solve(1, in, out);
        out.close();
    }

    static class inabameguru {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            Operation o = new Operation();
            int n = in.nextInt();
            Set<String> hs = new HashSet<>();
            for (int i = 0; i < n; i++) {
                String s = in.next();
                hs.add(s);
            }

            int[] mot = {97, 97, 101, 105, 117, 117};
            int[] chi = {0, 98, 103, 109, 110, 114};
            int[] res = new int[12];


            for (int i = 0; i < 6; i++) {
                res[2 * i] = chi[i];
                res[2 * i + 1] = mot[i];
            }
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 12; i++) {
                if (res[i] != 0) {
                    sb.append((char) res[i]);
                }
            }
            String cmp = sb.toString();
            if (!hs.contains(cmp)) {
                out.println(cmp);
                return;
            }


            while (o.nextPermutation(mot) != null) {
                chi[0] = 0;
                chi[1] = 98;
                chi[2] = 103;
                chi[3] = 109;
                chi[4] = 110;
                chi[5] = 114;
                while (o.nextPermutation(chi) != null) {
                    for (int i = 0; i < 6; i++) {
                        res[2 * i] = chi[i];
                        res[2 * i + 1] = mot[i];
                    }
                    sb = new StringBuilder();
                    for (int i = 0; i < 12; i++) {
                        if (res[i] != 0) {
                            sb.append((char) res[i]);
                        }
                    }
                    cmp = sb.toString();
                    if (!hs.contains(cmp)) {
                        out.println(cmp);
                        return;
                    }
                }
            }

            out.println("NO");
        }

    }

    static class Operation {
        public int[] nextPermutation(int[] array) {
            int k = -1;
            int i = array.length - 2;
            while (i >= 0) {
                if (array[i] < array[i + 1]) {
                    k = i;
                    break;
                }
                i--;
            }

            // 存在しない場合nullを返す
            if (k == -1) return null;
            int l = k + 1;
            i = array.length - 1;
            while (i > k + 1) {
                if (array[k] < array[i]) {
                    l = i;
                    break;
                }
                i--;
            }

            int tmp = array[k];
            array[k] = array[l];
            array[l] = tmp;
            int[] res = new int[array.length];
            // k+1 <-> n-1
            for (int j = k + 1; j < array.length + k - j; j++) {
                tmp = array[j];
                array[j] = array[array.length + k - j];
                array[array.length + k - j] = tmp;
            }
            return array;
        }

    }
}