n = read_line.to_i
s = (1..n).map { read_line }

ans = ""
while s.any? { |s| !s.empty? }
  pos = s.map(&.ljust(50, '~')).each_with_index.min[1]
  ans += s[pos][0]
  s[pos] = s[pos][1..]
end
puts ans