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

used = set()
st = lsA[0]
f = True
for i in range(1,N):
    if st == lsA[i]:
        continue
    else:
        if lsA[i] in used:
            f = False
        used.add(st)
        st = lsA[i]
if f:
    print(0)
    exit()

en = lsA[-1]
ll = []
while lsA and lsA[-1] == en:
    ll.append(lsA.pop())
lsB = ll + lsA
used = set()
st = lsB[0]
for i in range(1,N):
    if st == lsB[i]:
        continue
    else:
        if lsB[i] in used:
            print(-1)
            exit()
        used.add(st)
        st = lsB[i]
print(1)