#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)




string _s; int _n; int sa_k;
vector<int> Rank, tmp, sa, lcp;
bool cmp_sa(int i, int j) {
	if (Rank[i] != Rank[j]) return Rank[i]<Rank[j];
	return (i + sa_k <= _n ? Rank[i + sa_k] : -1)<(j + sa_k <= _n ? Rank[j + sa_k] : -1);
}
void construct_sa() {
	for (int i = 0; i <= _n; i++) { sa[i] = i; Rank[i] = i<_n ? _s[i] : -1; }
	for (sa_k = 1; sa_k <= _n; sa_k *= 2) {
		sort(sa.begin(), sa.end(), cmp_sa); tmp[sa[0]] = 0;
		for (int i = 1; i <= _n; i++) tmp[sa[i]] = tmp[sa[i - 1]] + (cmp_sa(sa[i - 1], sa[i]) ? 1 : 0);
		for (int i = 0; i <= _n; i++) Rank[i] = tmp[i];
	}
}
void construct_lcp() {
	for (int i = 0; i <= _n; i++) Rank[sa[i]] = i;
	int h = 0; lcp[0] = 0;
	for (int i = 0; i<_n; i++) {
		int j = sa[Rank[i] - 1]; if (h>0) h--;
		for (; j + h<_n&&i + h<_n; h++) if (_s[j + h] != _s[i + h]) break; lcp[Rank[i] - 1] = h;
	}
}
void init(string s) {
	_s = s;	_n = s.length();
	Rank = vector<int>(_n + 1); tmp = vector<int>(_n + 1);
	sa = vector<int>(_n + 1); lcp = vector<int>(_n + 1);
	construct_sa(); construct_lcp();
}
bool chk(string s) {
	int lo = 0, hi = _n + 1;
	while (lo + 1 != hi) {
		int md = (lo + hi) / 2;
		if (s <= _s.substr(sa[md], s.length())) hi = md;
		else lo = md;
	}
	
	string t = _s.substr(sa[hi], s.length());
	return s == t;
}
//-----------------------------------------------------------------------------------
int N;
string S[101010];
//-----------------------------------------------------------------------------------
#define rrep(i,a,b) for(int i=a;i>=b;i--)
string nxt(string x) {
	int carry = 1;
	rrep(i, x.length() - 1, 0) {
		int n = x[i] - 'a' + carry;
		carry = 0;
		if (26 <= n) {
			n %= 26;
			carry = 1;
		}
		x[i] = 'a' + n;
	}
	if (carry) x = "a" + x;
	return x;
}
//-----------------------------------------------------------------------------------
int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	cin >> N; rep(i, 0, N) cin >> S[i];

	string SS = "";
	rep(i, 0, N) SS += "#" + S[i];
	init(SS);

	string ans = "a";
	while (chk(ans)) ans = nxt(ans);
	cout << ans << endl;
}