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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
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 aa
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 a

import java.io.IOException;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.io.InputStream;

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

class TaskB {
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n=in.readInt();
        String name[]=new String[n];
        for (int i=0;i<n;i++) name[i]=in.readLine();
        String ans= "a";
        while (true){
            boolean flag= true;
            for (int j=0;j<n;j++){
                if (name[j].contains(ans)){
                    flag=false;
                    break;
                }
            }
            if (flag) {
                out.println(ans);
                return;
            }
            ans=next(new StringBuilder(ans));
        }
    }
    String next(StringBuilder str){
        int carry=1;
        for (int i=str.length()-1;i>=0;i--){
            char c= str.charAt(i);
            c+=carry;
            if (c>'z') c= 'a';
            else carry=0;
            str.setCharAt(i,c);
        }
        if (carry == 1) return "a" + str.toString();
        else return str.toString();
    }
}

class InputReader {

        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
                this.stream = stream;
        }

        public int read() {
                if (numChars == -1)
                        throw new InputMismatchException();
                if (curChar >= numChars) {
                        curChar = 0;
                        try {
                                numChars = stream.read(buf);
                        } catch (IOException e) {
                                throw new InputMismatchException();
                        }
                        if (numChars <= 0)
                                return -1;
                }
                return buf[curChar++];
        }

        public int readInt() {
                int c = read();
                while (isSpaceChar(c))
                        c = read();
                int sgn = 1;
                if (c == '-') {
                        sgn = -1;
                        c = read();
                }
                int res = 0;
                do {
                        if (c < '0' || c > '9')
                                throw new InputMismatchException();
                        res *= 10;
                        res += c - '0';
                        c = read();
                } while (!isSpaceChar(c));
                return res * sgn;
        }

        public boolean isSpaceChar(int c) {
                if (filter != null)
                        return filter.isSpaceChar(c);
                return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        private String readLine0() {
                StringBuilder buf = new StringBuilder();
                int c = read();
                while (c != '\n' && c != -1) {
                        if (c != '\r')
                                buf.appendCodePoint(c);
                        c = read();
                }
                return buf.toString();
        }

        public String readLine() {
                String s = readLine0();
                while (s.trim().length() == 0)
                        s = readLine0();
                return s;
        }

        public interface SpaceCharFilter {
                public boolean isSpaceChar(int ch);
        }
}

