TIOJ 2154

#include <bits/stdc++.h>
#include <time.h>
using namespace std;
#define SZ(x) (int)(x.size())
#define int long long

bool cnt[26], aux[26];

int32_t main(){
    srand(time(0));
    int n;
    cin >> n;
    string s;
    cin >> s;
    for(int i = 0; i < SZ(s); ++i)cnt[s[i]-'a'] = 1;
    for(int i = 1; i < n; ++i){
        cin >> s;
        memset(aux, 0, sizeof(aux));
        for(int j = 0; j < SZ(s); ++j){
            aux[s[j]-'a'] = 1;
        }
        for(int j = 0; j < 26; ++j){
            cnt[j] &= aux[j];
        }
    }
    int ans = 7122;
    for(int i = 0; i < 26 && ans == 7122; ++i){
        if(cnt[i] == 1)ans = i;
    }
    if(ans == 7122)cout << ans << '\n';
    else cout << char(ans + 'a') << '\n';
    return 0;
}