n = int(input())
a = list(map(int, input().split()))
s = [0] * n
s[a[0] - 1] = 1
for i in range(n - 1):
    if a[i + 1] != a[i]:
        s[a[i + 1] - 1] += 1
if a[0] == a[-1]:
    if n - s.count(0) - s.count(1) > 1 or n - s.count(0) - s.count(1) - s.count(2) > 0:
        print(-1)
    elif n - s.count(0) - s.count(1) == 1:
        print(1)
    else:
        print(0)
else:
    if n - s.count(0) - s.count(1) > 0:
        print(-1)
    else:
        print(0)