require "set"

N = gets.to_i
T = gets.split.map(&:to_i)

cands = (0..11).filter { |x|
  Set.new(T).subset? Set.new([x, x + 2, x + 4, x + 5, x + 7, x + 9, x + 11].map{_1 % 12})
}

puts cands.empty? || cands.size > 1 ? -1 : cands[0]