n = int(input())
l = []
for i in range(n):
    a = input()
    l.append(a)
for i in range(ord('a'),ord('z')+1):
    i = chr(i)
    g = 0
    for j in range(n):
        if i in l[j]:
            g = g + 1
    if g == n:
        print(i)
        break