3
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 1
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 30
b
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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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import java.util.*;
public class conteest {

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        int x = sc.nextInt();
        String[] m = new String[x];
        for (int i = 0; i < x; i++) {
            m[i] = sc.next();
        }
        String om = "";
        char[] xm = {'a', 'b', 'c', 'd', 'e','f', 'g', 'h','i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        String mx = "";
        String[] c = new String[702];
        for (int i = 0; i < 26; i++) {
            c[i] = "" + xm[i];
        }
//        for (int k = 26; k < 702; k++) {
            String abo="";
            int cv=26;
            for (int i = 0; i < 26; i++) {
                abo=""+xm[i];
                for (int j = 0; j < 26; j++) {
                    abo+=xm[j];
                    c[cv]=abo;
                    cv++;
                    abo=""+xm[i];
                }
                
            }
//        }
            boolean b=false;
            for(int i=0;i<720;i++){
                for(int j=0;j<x;j++){
                    if(m[j].contains(c[i])){
                      b=true;  
                    }
                }
                if(b==false){
                    System.out.println(c[i]);
                    break;
                }
                b=false;
            }
    }
}
