#include <bits/stdc++.h>
using namespace std;
const double ep = 1e-9;
const long long M = 1000010;
const long long IN = 1e15;
char str[35][25];
long long pr[M];
int main() {
  long long n, a, b, k;
  while (scanf("%lld", &n) != EOF) {
    for (long long i = 0; i < n; i++) {
      scanf("%s", str[i]);
    }
    bool flag = false, fflag = false;
    char sstr[5];
    for (char j = 'a'; j <= 'z' && !fflag; j++) {
      flag = true;
      sstr[0] = j;
      sstr[1] = '\0';
      for (long long i = 0; i < n; i++) {
        if (strstr(str[i], sstr) != 0) {
          flag = false;
        }
      }
      if (flag) {
        printf("%s\n", sstr);
        fflag = true;
      }
    }
    for (char k = 'a'; k <= 'z' && !fflag; k++) {
      for (char j = 'a'; j <= 'z' && !fflag; j++) {
        flag = true;
        sstr[0] = k;
        sstr[1] = j;
        sstr[2] = '\0';
        for (long long i = 0; i < n; i++) {
          if (strstr(str[i], sstr) != 0) {
            flag = false;
          }
        }
        if (flag) {
          printf("%s\n", sstr);
          fflag = true;
        }
      }
    }
  }
  return 0;
}
