n = int(input())
A = list(map(int, input().split()))
pre = -1
kouho = set()
out = []
for i in range(n):
    if A[i] == pre:
        continue
    if A[i] in kouho:
        out.append(A[i])
        pre = A[i]
        continue
    kouho.add(A[i])
    pre = A[i]
if not out:
    ans = 0
else:
    if len(out) == 1 and A[0] == A[-1]:
        ans = 1
    else:
        ans = -1
print(ans)