N = gets.to_i
S = (0 ... N).map { gets.chomp }
a = ?a.ord
S.map! { |s|
	counts = [0] * 26
	s.each_byte do |b|
		counts[b - a] += 1
	end
	counts
}

indeg = [0] * N
(0 ... N).each do |u|
	(u + 1 ... N).each do |v|
		if (0 ... 26).all? { |c| S[u][c] <= S[v][c] }
			indeg[v] += 1
		end
	end
end

if (i = indeg.index(0))
	puts (0 ... 26).sum("") { |c| (a + c).chr * S[i][c] }
else
	puts -1
end