N = int(input())
S = set()
for i in range(N):
    S.add(tuple(input()))
    
for i in range(1 << min(N, 10)):
    s = []
    for j in range(min(N, 10)):
        if (i >> j) & 1:
            s.append("a")
        else:
            s.append("b")
    while len(s) < N:
        s.append("a")
    s = tuple(s)
    if s not in S:
        print(*s, sep="")
        exit()