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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 1
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#include <bits/stdc++.h>
using namespace std;
const int MAXN = 40;
char word[MAXN][MAXN];
int N;
char ans[MAXN], tmp[MAXN];
bool found;
bool mystrstr(char* s, char* t) {
  int lens = strlen(s), lent = strlen(t);
  if (lens < lent) return false;
  for (int i = 0; i < lens; i++) {
    int length = 0, j = 0;
    while (j < lent && s[i] == t[j]) {
      length++;
      j++;
    }
    if (length == lent) return true;
  }
  return false;
}
bool Find(char* str) {
  for (int i = 1; i <= N; i++) {
    if (strstr(word[i], str)) {
      return true;
    }
  }
  return false;
}
void dfs(int depth, int maxdepth) {
  if (found) return;
  if (depth > maxdepth) return;
  if (depth == maxdepth) {
    tmp[depth] = '\0';
    if (Find(tmp) == false) {
      found = true;
      strcpy(ans, tmp);
    }
    return;
  }
  for (int i = 0; i < 26; i++) {
    tmp[depth] = i + 'a';
    dfs(depth + 1, maxdepth);
  }
}
int main() {
  while (scanf("%d", &N) != EOF) {
    for (int i = 1; i <= N; i++) scanf("%s", word[i]);
    int maxlen = 0;
    for (int i = 1; i <= N; i++) maxlen = max(maxlen, (int)strlen(word[i]));
    for (int i = 0; i < maxlen + 1; i++) ans[i] = 'a';
    ans[maxlen + 1] = '\0';
    found = false;
    for (int i = 1; i <= maxlen; i++) {
      if (found) break;
      strcpy(tmp, "");
      dfs(0, i);
    }
    printf("%s\n", ans);
  }
  return 0;
}
