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/*
ID: czq1992
LANG: C++
TASK: B
*/
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
using namespace std;

bool hash[1000];
int main()
{
    int i,j,k,t,len;
    int f;
    int code;
    char s[25];
    cin>>t;
    while(t--)
    {
        cin>>s;
        len=strlen(s);
        for(i=0; i<len; i++)
        {
            hash[s[i]-'a'+1]=1;
            if(i+1<len)
            {
                code=(s[i]-'a'+1)*26+s[i+1]-'a'+1;
                hash[code]=1;
            }
        }

    }
    for(i=1; i<703; i++)
    {
        if(!hash[i])
        {
            if(i>26) printf("%c%c\n",i/26+'a'-1,i%26+'a'-1);
            else printf("%c\n",i+'a'-1);
            break;
        }
    }
    return 0;
}
