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

d = {}
prev = -1
for a in A:
    if prev != a:
        d.setdefault(a, 0)
        d[a] += 1

t = len([i for i in d.values() if i >= 2])
if t >= 2:
    print(-1)
elif t == 1:
    if A[0] == A[-1]:
        print(1)
    else:
        print(-1)
else:
    print(0)