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#include <bits/stdc++.h>
using namespace std;
int alp[26] = {0};
int arr[26][26] = {0};
void fillArr1(string str) {
  int i = 0;
  int k;
  for (i = 0; i < str.length(); i++) {
    k = str[i];
    k = k - 97;
    alp[k]++;
  }
}
void fillArr2(string str) {
  int i, ri, ci;
  char r, c;
  for (i = 0; i < str.length() - 1; i++) {
    r = str[i];
    c = str[i + 1];
    ri = r;
    ri = ri - 97;
    ci = c;
    ci = ci - 97;
    arr[ri][ci]++;
  }
}
void fillArr(string str) {
  fillArr1(str);
  fillArr2(str);
}
int main() {
  int n;
  int i, j;
  string str;
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    cin >> str;
    fillArr(str);
  }
  for (i = 0; i < 26; i++) {
    if (alp[i] == 0) {
      break;
    }
  }
  if (i < 26) {
    char ch = i + 97;
    printf("%c", ch);
    return 0;
  } else {
    for (i = 0; i < 26; i++) {
      for (j = 0; j < 26; j++) {
        if (arr[i][j] == 0) {
          char ch1 = i + 97;
          char ch2 = j + 97;
          printf("%c%c", ch1, ch2);
          return 0;
        }
      }
    }
  }
  return 0;
}
