func main()
	var n: int :: cui@inputInt()
	var t: []int :: #[n]int
	for i(0, n - 1)
		do t[i] :: cui@inputInt()
	end for
	
	var ans: int :: -1
	var dd: []int :: [0, 2, 4, 5, 7, 9, 11]
	for d(0, 11)
		var f: bool :: true
		for i(0, n - 1)
			for j(0, 6)
				if(t[i]= (dd[j] + d) % 12)
					skip i
				end if
			end for
			do f :: false
			break i
		end for
		
		if(f)
			if(ans = -1)
				do ans :: d
			else
				do ans :: -1
				break d
			end if
		end if
	end for
	do cui@print("\{ans}\n")
end func