n = int(input())
a = list(map(int, input().split()))
idx = 0
while idx < n and a[idx] == a[-1]:
    a.append(a[0])
    idx += 1
s = set([a[idx]])
for i in range(idx, n - 1 + idx):
    if a[i] != a[i + 1]:
        if a[i + 1] in s:
            print(-1)
            break
else:
    if idx == 0 or idx == n:
        print(0)
    else:
        print(1)