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

d = {}
prev = ''
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:
    print(1)
else:
    print(0)