N=int(input())
A=list(map(int,input().split()))
B=[False]*N
P=0
if len(set(A))==1:
  print(0)
  exit()
if A[0]==A[-1]:
  for i in range(N-1,-1,-1):
    if A[0]==A[i]:
      del A[i]
    else:
      break
  P=1
for i in range(len(A)-1):
  B[A[i]-1]=True
  if B[A[i+1]-1] and (A[i]!=A[i+1]):
    print(-1)
    exit()
print(P)