n = gets.to_i
a = gets.split.map &:to_i
s = a.uniq.size
if s == n
	puts 0
elsif a.first == a.last
	b = a[a.index { |i| i != a.first }..a.rindex { |i| i != a.last }]
	puts b.uniq.size == b.size ? 1 : -1
else
	puts -1
end