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

cands = (0..11).select { |d|
  ham = [d, d + 2, d + 4, d + 5, d + 7, d + 9, d + 11].map { |v| v % 12 }

  (T - ham).empty?
}

if cands.size == 1
  puts cands.first
else
  puts -1
end