a = {0, 2, 4, 5, 7, 9, 11}
n = io.read("*n")
t = {}
for i = 1, n do
  z = io.read("*n")
  t[z] = true
end
r = -1
for i = 0, 11 do
  f = true
  y = {}
  for j = 1, #a do
    z = (i + a[j]) % 12
    y[z] = true
  end
  for k, v in pairs(t) do
    if not y[k] then f = false break end
  end
  if f then
    if r == -1 then r = i
    else r = -1 break
    end
  end
end
print(r)