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import sys, math
def rs():
    return sys.stdin.readline().strip()
def ri():
    return int(sys.stdin.readline().strip())
def ras():
    return list(sys.stdin.readline().strip())
def rai():
    return map(int,sys.stdin.readline().strip().split())

def main():
    n = ri()
    _set = set([])
    for x in xrange(n):
        s = ras()
        _set.add(s[0])
        for y in xrange(1, len(s)):
            _set.add(s[y])
            _set.add(s[y-1]+s[y])

    abc = list("qwertyuiopasdfghjklzxcvbnm")
    alls = []

    for x in abc:
        alls.append(x)

    for x in abc:
        for y in abc:
            alls.append(x+y)


    def comp(x,y):
        if len(x) != len(y):
            return len(x) - len(y)
        if x > y:
            return 1
        elif y > x:
            return -1
        else:
            return 0

    alls = sorted(alls, cmp=comp )

    for x in alls:
        if x not in _set:
            print x
            return




if __name__ == "__main__":
    main()