#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l_temp, C[1000];
  char A[5], temp[21];
  vector<char> B, D;
  bool good;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%s", temp);
    l_temp = strlen(temp);
    for (int j = 0; j < l_temp; j++) B.push_back(temp[j]);
    B.push_back('#');
  }
  for (int i = 0; i < B.size(); i++) B[i] -= 'a';
  for (int i = 0; i < 26 && !good; i++) {
    D.clear();
    D.push_back(i);
    D.push_back('#');
    for (int p = 0; p < B.size(); p++) D.push_back(B[p]);
    C[0] = 0;
    int d = 0;
    for (int p = 1; p < D.size(); p++) {
      while (d > 0 && D[p] != D[d]) d = C[d - 1];
      if (D[p] == D[d]) d++;
      C[p] = d;
    }
    good = true;
    for (int p = 0; p < D.size(); p++)
      if (C[p] == 1) good = false;
    if (good) printf("%c\n", i + 'a');
  }
  for (int i = 0; i < 26 && !good; i++)
    for (int j = 0; j < 26 && !good; j++) {
      D.clear();
      D.push_back(i);
      D.push_back(j);
      D.push_back('#');
      for (int p = 0; p < B.size(); p++) D.push_back(B[p]);
      C[0] = 0;
      int d = 0;
      for (int p = 1; p < D.size(); p++) {
        while (d > 0 && D[p] != D[d]) d = C[d - 1];
        if (D[p] == D[d]) d++;
        C[p] = d;
      }
      good = true;
      for (int p = 0; p < D.size(); p++)
        if (C[p] == 2) good = false;
      if (good) printf("%c%c\n", i + 'a', j + 'a');
    }
  for (int i = 0; i < 26 && !good; i++)
    for (int j = 0; j < 26 && !good; j++)
      for (int k = 0; k < 26 && !good; k++) {
        D.clear();
        D.push_back(i);
        D.push_back(j);
        D.push_back(k);
        D.push_back('#');
        for (int p = 1; p < B.size(); p++) D.push_back(B[p]);
        C[0] = 0;
        int d = 0;
        for (int p = 0; p < D.size(); p++) {
          while (d > 0 && D[p] != D[d]) d = C[d - 1];
          if (D[p] == D[d]) d++;
          C[p] = d;
        }
        good = true;
        for (int p = 0; p < D.size(); p++)
          if (C[p] == 3) good = false;
        if (good) printf("%c%c%c\n", i + 'a', j + 'a', k + 'a');
      }
  return 0;
}
