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 30
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 2
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#include <bits/stdc++.h>
using namespace std;
int N;
string S[22];
bool vst[28];
struct node {
  int i, j;
};
vector<node> V[28];
int main() {
  int T, cas = 0;
  int i, j;
  scanf("%d", &N);
  for (i = 0; i < N; i++) cin >> S[i];
  int sz, a;
  node z;
  for (i = 0; i < N; i++) {
    sz = S[i].size();
    for (j = 0; j < sz; j++) {
      a = S[i][j] - 'a';
      vst[a] = 1;
      z.i = i;
      z.j = j;
      V[a].push_back(z);
    }
  }
  for (i = 0; i < 26; i++) {
    if (!vst[i]) {
      printf("%c\n", i + 'a');
      return 0;
    }
  }
  int k;
  for (i = 0; i < 26; i++) {
    bool t = 0;
    for (j = 0; j < 26; j++) {
      for (k = 0; k < V[i].size(); k++) {
        int a = V[i][k].i;
        int b = V[i][k].j;
        if (S[a][b + 1] == j + 'a') {
          t = 1;
          break;
        }
        printf("%c%c\n", i + 'a', j + 'a');
        return 0;
      }
    }
  }
  printf("aaa\n");
  return 0;
}
