#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  char M[31][31];
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> M[i];
  }
  for (int i = 97; i <= 122; i++) {
    bool f = true;
    for (int j = 0; j < n; j++) {
      for (int k = 0; k < strlen(M[j]); k++) {
        if (int(M[j][k]) == i) f = false;
      }
    }
    if (f) {
      cout << char(i) << endl;
      return 0;
    }
  }
  for (int i = 97; i <= 122; i++) {
    for (int x = 97; x <= 122; x++) {
      bool f = true;
      for (int j = 0; j < n; j++) {
        for (int k = 0; k < strlen(M[j]) - 1; k++) {
          if (int(M[j][k]) == i && int(M[j][k + 1]) == x) f = false;
        }
      }
      if (f) {
        cout << char(i) << char(x) << endl;
        return 0;
      }
    }
  }
  for (int i = 97; i <= 122; i++) {
    for (int x = 97; x <= 122; x++) {
      for (int y = 97; y <= 122; y++) {
        bool f = true;
        for (int j = 0; j < n; j++) {
          for (int k = 0; k < strlen(M[j]) - 2; k++) {
            if (int(M[j][k]) == i && int(M[j][k + 1]) == x &&
                int(M[j][k + 2]) == y)
              f = false;
          }
        }
        if (f) {
          cout << char(i) << char(x) << char(y) << endl;
          return 0;
        }
      }
    }
  }
  for (int i = 97; i <= 122; i++) {
    for (int x = 97; x <= 122; x++) {
      for (int y = 97; y <= 122; y++) {
        for (int o = 97; o <= 122; o++) {
          bool f = true;
          for (int j = 0; j < n; j++) {
            for (int k = 0; k < strlen(M[j]) - 3; k++) {
              if (int(M[j][k]) == i && int(M[j][k + 1]) == x &&
                  int(M[j][k + 2]) == y && int(M[j][k + 3]) == o)
                f = false;
            }
          }
          if (f) {
            cout << char(i) << char(x) << char(y) << char(o) << endl;
            return 0;
          }
        }
      }
    }
  }
  return 0;
}
