def solve():
    from random import randint

    N=int(input())

    NG=[None]*N
    for i in range(N):
        NG[i]=input()

    while True:
        X=["a" if randint(0,1)==0 else "b" for _ in range(N)]
        X="".join(X)
        if X not in NG:
            return X

#==================================================
print(solve())