#include <bits/stdc++.h>
using namespace std;
int a, c, d[205], f, n, b, x, i;
string s, s1, s11, s12;
char i1, i2, j;
int main() {
  cin >> n;
  for (i = 1; i <= n; i++) {
    cin >> s1;
    b = s1.size();
    c = c + b;
    s = s + '%';
    s = s + s1;
  }
  c = c + n - 1;
  for (i1 = 'a'; i1 <= 'z'; i1++) {
    s1 = i1;
    x = s.find(s1);
    if (x == -1) {
      cout << s1;
      return 0;
    }
  }
  for (i1 = 'a'; i1 <= 'z'; i1++) {
    for (i2 = 'a'; i2 <= 'z'; i2++) {
      s11 = i1;
      s12 = i2;
      s1 = s11 + s12;
      x = s.find(s1);
      if (x == -1) {
        cout << s1;
        return 0;
      }
    }
  }
}
