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//package codeforces;

import java.io.BufferedReader;
import java.io.Closeable;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.StringTokenizer;

public class B implements Closeable {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    PrintWriter writer = new PrintWriter(System.out);
    StringTokenizer stringTokenizer;

    B() throws IOException {
//        reader = new BufferedReader(new FileReader("input.txt"));
//        writer = new PrintWriter(new FileWriter("output.txt"));
    }

    String next() throws IOException {
        while (stringTokenizer == null || !stringTokenizer.hasMoreTokens()) {
            stringTokenizer = new StringTokenizer(reader.readLine());
        }
        return stringTokenizer.nextToken();
    }

    int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    long nextLong() throws IOException {
        return Long.parseLong(next());
    }

    double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    final int MOD = 1000 * 1000 * 1000 + 7;

    int sum(int a, int b) {
        a += b;
        return a >= MOD ? a - MOD : a;
    }

    int product(int a, int b) {
        return (int) (1l * a * b % MOD);
    }

    int pow(int x, long k) {
        int result = 1;
        while (k > 0) {
            if (k % 2 == 1) {
                result = product(result, x);
            }
            x = product(x, x);
            k /= 2;
        }
        return result;
    }

    @SuppressWarnings("unchecked")
    void solve() throws IOException {
        class Node {
            char c;
            Node parent;
            Node[] children = new Node[26];

            public Node(char c, Node parent) {
                this.c = c;
                this.parent = parent;
            }

            Node createChild(char c) {
                int ci = c - 'a';
                if(children[ci] == null) {
                    children[ci] = new Node(c, this);
                }
                return children[ci];
            }
        }
        final Node root = new Node('r', null);
        int n = nextInt();
        for(int k = 0; k < n; k++) {
            char[] s = next().toCharArray();
            for(int i = 0; i < s.length; i++) {
                Node node = root;
                for(int j = i; j < s.length; j++) {
                    node = node.createChild(s[j]);
                }
            }
        }
        class Utils {
            void print(Node node) {
                if(node == root) {
                    return;
                }
                print(node.parent);
                writer.print(node.c);
            }
        }
        Queue<Node> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()) {
            Node node = q.poll();
            for(int i = 0; i < 26; i++) {
                if(node.children[i] == null) {
                    new Utils().print(node.createChild((char)('a' + i)));
                    return;
                }
                q.add(node.children[i]);
            }
        }
    }

    public static void main(String[] args) throws IOException {
        try (B b = new B()) {
            b.solve();
        }
    }

    @Override
    public void close() throws IOException {
        reader.close();
        writer.close();
    }
}