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

tmp = A[0]
st = {tmp}
ans = 0
for a in A:
    if a != tmp:
        tmp = a
        if ans:
            print(-1)
            break
        elif tmp in st:
            if a == A[0]:
                ans = 1
        else:
            st.add(tmp)
else:
    print(ans)