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
    prev = a

if any(v >= 3 for v in d.values()):
    print(-1)
else:
    t = len([v for v in d.values() if v >= 2])
    if t >= 2:
        print(-1)
    elif t == 1:
        if A[0] == A[-1]:
            print(1)
        else:
            print(-1)
    else:
        print(0)