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

if N == 1
  puts 0
  exit
end

rotate_cnt = 0

while A[0] == A[-1]
  rotate_cnt += 1
  a = A.shift
  A.push(a)
end

cnt = 0

(N - 1).times do |i|
  cnt += 1 if A[i] != A[i + 1]
end

if cnt == A.uniq.size - 1
  if rotate_cnt == 0
    puts 0
  else
    puts 1
  end
else
  puts -1
end