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

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