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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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 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>
int n;
int a[28][28][28];
char c[105];
int main() {
  int i, j, k, m, o;
  scanf("%d", &n);
  for (i = 1; i <= n; i++) {
    scanf("%s", &c[1]);
    m = strlen(&c[1]);
    for (j = 1; j <= m; j++) {
      a[c[j] - 'a' + 1][0][0] = 1;
      if (j != m) {
        a[c[j] - 'a' + 1][c[j + 1] - 'a' + 1][0] = 1;
        if (j != m - 1)
          a[c[j] - 'a' + 1][c[j + 1] - 'a' + 1][c[j + 2] - 'a' + 1] = 1;
      }
    }
  }
  o = 0;
  for (i = 1; i <= 26; i++)
    if (a[i][0][0] == 0) {
      o = 1;
      printf("%c", i - 1 + 'a');
      break;
    }
  for (i = 1; i <= 26; i++)
    for (j = 1; j <= 26; j++)
      if (a[i][j][0] == 0 && o == 0) {
        o = 1;
        printf("%c%c", i - 1 + 'a', j - 1 + 'a');
      }
  for (i = 1; i <= 26; i++)
    for (j = 1; j <= 26; j++)
      for (k = 1; k <= 26; k++)
        if (a[i][j][k] == 0 && o == 0) {
          o = 1;
          printf("%c%c%c", i - 1 + 'a', j - 1 + 'a', k - 1 + 'a');
        }
  return 0;
}
