3
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 1
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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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 5
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 2
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 1
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 30
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 30
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 1
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#include <bits/stdc++.h>
using namespace std;
char bufs[30][21];
char t1[26];
char t2[676][3];
int main() {
  int n, i, j;
  for (i = 0; i < 26; i++) t1[i] = 'a' + i;
  for (i = 0; i < 26; i++)
    for (j = 0; j < 26; j++) {
      t2[i * 26 + j][0] = 'a' + i;
      t2[i * 26 + j][1] = 'a' + j;
      t2[i * 26 + j][2] = '\0';
    }
  scanf("%d", &n);
  for (i = 0; i < n; i++) scanf("%s", bufs[i]);
  for (j = 0; j < 26; j++) {
    for (i = 0; i < n; i++) {
      if (strchr(bufs[i], t1[j])) break;
    }
    if (i == n) {
      printf("%c\n", t1[j]);
      break;
    }
  }
  if (j == 26) {
    for (j = 0; j < 676; j++) {
      for (i = 0; i < n; i++) {
        if (strstr(bufs[i], t2[j])) break;
      }
      if (i == n) {
        printf("%s\n", t2[j]);
        break;
      }
    }
  }
  return 0;
}
