n = int(input())
A = list(map(int, input().split()))

if len(set(A)) == 1:
    print(0)
    exit()

from collections import deque
B = deque(A)
temp = A[0]
flag = False
while True:
    x = B.pop()
    if x != temp:
        B.append(x)
        break
    else:
        B.appendleft(x)
        flag = True

if flag:
    ans = 1
else:
    ans = 0

S = set()
C = list(B)
temp = C[0]
for i in range(n):
    if C[i] != temp:
        if C[i] in S:
            print(-1)
            exit()
        else:
            S.add(temp)
            temp = C[i]
else:
    print(ans)