require 'set'
ans=[]
n=gets.to_i
ss=Set.new(gets.split(" ").map{|e| e.to_i})
12.times{|e|
	ss2=Set[]
	[0,2,4,5,7,9,11].each{|e2|
		ss2<<(e+e2)%12
	}
	if ss.size==(ss&ss2).size then
		ans<<e
	end
} 
if ans.size==1 then
	puts ans[0]
else
	puts -1
end