3
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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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a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <iostream>
#include <vector>
#include <string>
#include <deque>
#include <algorithm>
#include <functional>
#include <map>
#include <bitset>
#include <iomanip>
#include <list>
#include <set>
#include <deque>
using namespace std;
#define FOR(i,b,e) for( int i = b; i <  e; ++i )
#define FOI(i,b,e) for( int i = b; i <= e; ++i )
#define FOD(i,b,e) for( int i = b; i >= e; --i )
#define REP(c) FOR(_______,0,c)

#define sz(z) int( (z).size() )
#define all(a) (a).begin(), (a).end()
#define pii pair<int,int>
#define pt pair<int,int>
#define x first
#define y second
#define uint64 unsigned __int64
#define int64 __int64
#define wtf(s) {while(1)puts(s);}


void deb(){}

bool IsLexLess( string a, string b ){
    if( sz(a) < sz(b) && a == b.substr(0,sz(a)) ) return true;
    return a < b;
}

string getNext( string s ){
    int car = 1;
    FOD( j, sz(s)-1, 0 ){
        s[j] += car;
        car = 0;
        if( s[j] == 'z'+1 ){
            car = 1;
            s[j] = 'a';
        }
    }
    if( car ) s = 'a' + s;
    return s;
}

__int32 main(){
#ifndef ONLINE_JUDGE
    freopen("input.txt", "rt", stdin);
#endif

    int n;
    vector<string> s;

    cin >> n;
    REP(n){
        string b;
        cin >> b;
        FOR( i, 0, sz(b) ){
            FOR( j, i, sz(b) ){
                s.push_back( b.substr(i,j-i+1) );
            }
        }
    }

    sort( all(s) );
    s.erase( unique(all(s)), s.end() );
    
    string ans( 10000, 'z' );
    if( s[0] != "a" ) cout << "a" << endl;
    else{
        FOR( i, 0, sz(s) ){
            string to = getNext( s[i] );
            //cout << s[i] << " " << to << endl;
            if( !count(all(s),to) ){
                if( sz(ans) > sz(to) 
                    || sz(ans) == sz(to) && IsLexLess( to, ans )
                ){
                    ans = to;
                }
            }
        }
    }
    cout << ans << endl;
    return 0;
}