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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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#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, t, j, flag, sum, x, s;
  int b[111][111], num[111], d[111];
  char a[111][111];
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    scanf("%s", &a[i]);
    t = strlen(a[i]);
    num[i] = t;
    for (j = 0; j < t; j++) {
      b[i][j] = a[i][j] - 'a' + 1;
    }
  }
  d[0] = 1;
  flag = 1;
  t = 1;
  while (flag) {
    x = 0;
    for (i = 0; i < n; i++) {
      for (j = 0; j <= num[i] - t; j++) {
        sum = 0;
        for (s = t - 1; s >= 0; s--) {
          if (b[i][j + s] == d[s]) sum++;
        }
        if (sum == t) {
          x = 1;
          break;
        }
      }
    }
    if (x == 1) {
      d[0]++;
      for (i = 0; i < t; i++) {
        if (d[i] > 26) {
          d[i] = d[i] % 26;
          d[i + 1]++;
        }
      }
      if (d[t] == 1) t++;
    } else
      flag = 0;
  }
  for (i = t - 1; i >= 0; i--) {
    printf("%c", d[i] + 'a' - 1);
  }
  printf("\n");
}
