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#!/usr/bin/env python
import sys

def substring(s, k):
    for i in xrange(len(s) - k + 1):
        yield s[i:i+k]

def strings_of_size(k):
    'All strings of size k'
    if k == 0:
        yield ''
        return

    for c in "abcdefghijklmnopqrstuvwxyz":
        for d in strings_of_size(k-1):
            yield c + d

if __name__ == '__main__':
    n = int(sys.stdin.readline())
    substrings = set()

    for _ in xrange(n):
        s = sys.stdin.readline().strip()
        for k in xrange(1, len(s) + 1):
            for x in substring(s, k):
                substrings.add(x)

    found = False
    for k in xrange(1, 22):
        for s in strings_of_size(k):
            if s not in substrings:
                print s
                found = True
                break

        if found:
            break
        