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#!/usr/bin/python

def main():
    n = int(raw_input())
    s = []
    m1 = [0] * 200
    m2 = []
    for i in xrange(200):
        m2.append([0] * 200)
    for i in xrange(n):
        s.append(raw_input().strip())
    for t in s:
        for ch in t:
            m1[ord(ch)] = 1
        for i in xrange(len(t) - 1):
            m2[ord(t[i])][ord(t[i+1])] = 1
    for i in xrange(ord('a'), ord('z')+1):
        if m1[i] == 0:
            print "%c" % i
            return
    for i in xrange(ord('a'), ord('z')+1):
        for j in xrange(ord('a'), ord('z')+1):
            if m2[i][j] == 0:
                print "%c%c" % (i,j)
                return

if __name__ == "__main__":
    main()
