[백준] 1032

n = int(input())
l = []

for i in range(n):
  l.append(input())

s = ""
for i in range(len(l[0])):
  a = l[0][i]
  # print(a)
  t = 0 
  for j in range(n):
    if a != l[j][i]:
      t = 1
  if t == 0:
    s += a
  else:
    s += "?"

print(s)