n = int(input())
A = list(map(int, input().split()))
cnt = {}
B = []
for a in A:
    if not B or B[-1] != a:
        B.append(a)
        cnt[a] = cnt.get(a, 0) + 1

c = 0
for v in cnt.values():
    c += v - 1
if c == 0:
    print(0)
elif c == 1 and B[0] == B[-1]:
    print(1)
else:
    print(-1)