#include <bits/stdc++.h>
using namespace std;

#define int ll
#define fast_io cin.tie(0)->sync_with_stdio(0);
#define endl '\n'
typedef long long ll;

int32_t main() {
	fast_io;

	int n; cin >> n;
	set<string> st;
	for (int i = 0; i < n; i++) {
		string s; cin >> s;
		st.insert(s);
	}
	
	for (int i = 0; i <= n; i++) {
		string r;
		for (int j = 0, val = i; j < n; j++, val /= 2)
			r += 'a' + (val % 2);
		if (!st.count(r)) {
			cout << r << endl;
			return 0;
		}
	}

}