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import java.io.*;
import java.util.*;

/*

 */
public class A {
	static FastReader sc=null;
	
	public static void main(String[] args) {
		sc=new FastReader();
		int n=sc.nextInt();
		Set<String> set=new HashSet<>();
		for(int k=0;k<n;k++) {
			char line[]=sc.next().toCharArray();
			for(int i=0;i<line.length;i++) {
				StringBuilder sb=new StringBuilder();
				sb.append(line[i]);
				set.add(sb.substring(0));
				for(int j=i+1;j<line.length;j++) {
					sb.append(line[j]);
					set.add(sb.substring(0));
				}
			}
		}
		char a[]=new char[26];
		for(int i=0;i<26;i++)a[i]=(char)(i+'a');
		outer:for(int l=1;l<=n;l++) {
			for(int i=0;i+l<26;i++) {
				StringBuilder sb=new StringBuilder();
				for(int j=i;j<i+l;j++)sb.append(a[j]);
				if(!set.contains(sb.substring(0))) {
					System.out.println(sb.substring(0));
					break outer;
				}
			}
		}
		
		
		
	}
	
	
	
	

	
	
	
	
	
	
	static int[] reverse(int a[]) {
		ArrayList<Integer> al=new ArrayList<>();
		for(int i:a)al.add(i);
		Collections.sort(al,Collections.reverseOrder());
		for(int i=0;i<a.length;i++)a[i]=al.get(i);
		return a;
	}
	static int gcd(int a,int b) {
		if(b==0)return a;
		else return gcd(b,a%b);
	}
	static long gcd(long a,long b) {
		if(b==0)return a;
		else return gcd(b,a%b);
	}
	
	static void ruffleSort(int a[]) {
		ArrayList<Integer> al=new ArrayList<>();
		for(int i:a)al.add(i);
		Collections.sort(al);
		for(int i=0;i<a.length;i++)a[i]=al.get(i);
	}
	
	
	static void print(long a[]) {
		for(long e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	static void print(char a[]) {
		for(char e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	
	
	static void print(int a[]) {
		for(int e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	static void print(double a[]) {
		for(double e:a) {
			System.out.print(e+" ");
		}
		System.out.println();
	}
	
	static class FastReader 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
        int[] readArray(int n) {
    		int a[]=new int [n];
    		for(int i=0;i<n;i++) {
    			a[i]=sc.nextInt();
    		}
    		return a;
    	}
    } 
}






