# coding: utf-8
from collections import deque
N = int(input())
A = list(map(int,input().split()))

A = deque(A)
cnt = 0
while True:
    a1 = A.popleft()
    if a1 == A[-1]:
        A.append(a1)
        cnt += 1
    else:
        A = deque([a1]) + A
        break
#print(A)
L = [False] * (N+1)
a = [A[0]]
for i in range(N-1):
    if A[i+1] != A[i]:
        a.append(A[i+1])
    #print(A)
flg = True
for i in range(len(a)):
    if L[a[i]]:
        flg = False
    else:
        L[a[i]] = True

if flg:
    print(cnt)
else:
    print(-1)