import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
A=[int(i) for i in input().split()]
if n<=2:
  print(0)
elif len(set(A))==1:
  print(0)
else:
  chk=0
  while True:
    if A[-1]==A[0]:
      A.pop()
      chk=1
    else:
      break
  prev=0
  C=set()
  for i,a in enumerate(A):
    if a==prev:
      continue
    else:
      if a in C:
        print(-1)
        break
      else:
        C.add(a)
        prev=a
  else:
    print(chk)