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

int main()
{
	ios::sync_with_stdio(false), cin.tie(0);
	int N;
	cin >> N;
	vector<string> S(N);
	set<string> st;
	for (int i = 0; i < N; i++) {
		cin >> S[i];
		int n = S[i].size();
		for (int l = 0; l <= 4; l++) {
			for (int j = 0; j + l <= n; j++) {
				st.insert(S[i].substr(j, l));
			}
		}
	}
	string s = "a";
	for (s[0] = 'a'; s[0] <= 'z'; s[0]++) {
		if (st.count(s) == 0) {
			cout << s << endl;
			return 0;
		}
	}
	s = "aa";
	for (s[0] = 'a'; s[0] <= 'z'; s[0]++) {
		for (s[1] = 'a'; s[1] <= 'z'; s[1]++) {
			if (st.count(s) == 0) {
				cout << s << endl;
				return 0;
			}
		}
	}
	s = "aaa";
	for (s[0] = 'a'; s[0] <= 'z'; s[0]++) {
		for (s[1] = 'a'; s[1] <= 'z'; s[1]++) {
			for (s[2] = 'a'; s[2] <= 'z'; s[2]++) {
				if (st.count(s) == 0) {
					cout << s << endl;
					return 0;
				}
			}
		}
	}
	s = "aaaa";
	for (s[0] = 'a'; s[0] <= 'z'; s[0]++) {
		for (s[1] = 'a'; s[1] <= 'z'; s[1]++) {
			for (s[2] = 'a'; s[2] <= 'z'; s[2]++) {
				for (s[3] = 'a'; s[3] <= 'z'; s[3]++) {
					if (st.count(s) == 0) {
						cout << s << endl;
						return 0;
					}
				}
			}
		}
	}
	return 0;
}