#line 1 "template/template.h"
#include <bits/stdc++.h>
using namespace std;

#line 1 "template/misc.h"
/* --- PBDS, reference: https://codeforces.com/blog/entry/11080 --- */
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<class T> using IndexSet=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
template<class T> using IndexMultiset=tree<T,null_type,less_equal<T>,rb_tree_tag,tree_order_statistics_node_update>;
/* --- CUSTOM HASH, reference: https://codeforces.com/blog/entry/62393 --- */
struct MyHash {
    static uint64_t splitmix64(uint64_t x){x+=0x9e3779b97f4a7c15;x=(x^(x>>30))*0xbf58476d1ce4e5b9;x=(x^(x>>27))*0x94d049bb133111eb;return x^(x>>31);}
    size_t operator()(uint64_t x) const{static const uint64_t FIXED_RANDOM=chrono::steady_clock::now().time_since_epoch().count();return splitmix64(x+FIXED_RANDOM);}
};
template<class T> using us=unordered_set<T, MyHash>;template<class T,class U> using um=unordered_map<T,U,MyHash>;
using ll=long long;using ull=unsigned long long;using ld=long double;
const int MOD7=1000000007;
const int MOD3=998244353;
const ll dx[]{+0,+1,+0,-1,+1,+1,-1,-1};
const ll dy[]{+1,+0,-1,+0,+1,-1,-1,+1};
const ll INF64=0x1fffffffffffffff;
template<class T> using v=vector<T>;template<class T> using vv=v<v<T>>;template<class T> using vvv=v<vv<T>>;
using vl=v<ll>;using vvl=vv<ll>;using vi=v<int>;using vvi=vv<int>;using vs=v<string>;
using vb=v<bool>;using vd=v<ld>;using pl=pair<ll,ll>;using vp=v<pl>;using pd=pair<ld,ld>;using vpd=v<pd>;
template<class T> using pqg=priority_queue<T,vector<T>,greater<T>>;
#line 1 "template/func.h"
#define overload3(a, b, c, name, ...) name
#define rep1(n) for(ll i=0;i<n;++i)
#define rep2(i, n) for(ll i=0;i<n;++i)
#define rep3(i, a, b) for(ll i=a;i<b;++i)
#define rep(...) overload3(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep1(n) for(ll i=n;i--;)
#define rrep2(i, n) for(ll i=n;i--;)
#define rrep3(i, a, b) for(ll i=b;i-->(a);)
#define rrep(...) overload3(__VA_ARGS__,rrep3,rrep2,rrep1)(__VA_ARGS__)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define sm(...) accumulate(all(__VA_ARGS__),0LL)
#define CASE ll _t; cin >> _t; while (_t--)
#define debug(v) cout << "Line(" << __LINE__ << ") -> " << #v << " = " << (v) << endl;
#define YESNO(yes,no)void yes(bool i=1){cout<<(i?#yes:#no)<<'\n';}void no(){cout<<(#no)<<'\n';}
YESNO(first,second)YESNO(First,Second)YESNO(Yes,No)YESNO(YES,NO)YESNO(possible,impossible)YESNO(POSSIBLE,IMPOSSIBLE)
vector<ll> iota(ll n,ll begin=0){vector<ll>a(n);iota(all(a),begin);return a;}
vector<ll> divisors(ull x){vector<ll>ans;for(ull i=1;i*i<=x;i++)if(x%i==0){ans.push_back(i);}for(int i=(ans.size()-(ans.back()*ans.back()==x));i--;){ans.push_back(x/ans[i]);}return ans;}
vector<pl> factorize(ull x){vector<pl>ans;for(ull i=2;i*i<=x;i++)if(x%i==0){ans.push_back({i,1});while((x/=i)%i==0)ans.back().second++;}if(x!=1)ans.push_back({x,1});return ans;}
ll pow_mod(ll x,ll t,ll mod){if(!t)return 1;x%=mod;ll r=pow_mod(x,t/2,mod);(r*=r)%=mod;if(t%2)(r*=x)%=mod;return r;}
ll popcnt(ull a){return __builtin_popcountll(a);}
string operator*(const string s, int x) { string next; rep(x) next += s; return next; }
template<class T>void distinct(T&a){sort(all(a));a.erase(unique(all(a)),end(a));}
template<class T,class F>void filter(T&a,F b){a.erase(remove_if(all(a),not_fn(b)),a.end());}
template<class T> auto press(const T &a) { vector<pair<decay_t<decltype(a[0])>, ll>> ans; for(auto&&x:a) { if (ans.empty() || ans.back().first != x) ans.emplace_back(x, 1); else ans.back().second++; } return ans; }
template<class T>bool chmin(T&a,const T&b){if(a<=b)return 0;a=b;return 1;}template<class T,class U>bool chmin(T&a,const U&b){return chmin(a,(T)b);}
template<class T>bool chmax(T&a,const T&b){if(a>=b)return 0;a=b;return 1;}template<class T,class U>bool chmax(T&a,const U&b){return chmax(a,(T)b);}
#line 1 "template/io.h"
template<typename T,typename U>ostream&operator<<(ostream&os,pair<T,U>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T,typename U>istream&operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<typename T>ostream&operator<<(ostream&os,vector<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>istream&operator>>(istream&is,vector<T>&v){for(T&e:v)is>>e;return is;}
struct Nyan{Nyan(){cin.tie(nullptr)->sync_with_stdio(0);fixed(cout).precision(12);}}NYAAN;
#line 2 "main.cpp"

signed main() {
    ll n; cin >> n;
    vs g(n); cin >> g;

    string ans(n, 'a');

    unordered_set<string> seen;
    rep(n) { seen.insert(g[i]); }
    function<bool(string&)> chk = [&](string&s) -> bool {
        return !seen.count(s);
    };

    for (int i = 0; i < (1ll << n); i++) {
        string tmp = ans;
        for (int j = 0; j < n; j++) {
            if ((i >> j) & 1) {
                tmp[j] = 'b';
            }
        }
        if (chk(tmp)) {
            ans = tmp;
            break;
        }
    }
    cout << ans << '\n';
}