N = int(input())
A = list(map(int,input().split()))
X = [0] * (N + 1)
start = A[0]
for i in range(N - 1):
    if A[i] != A[i+1]:
        if X[A[i]] == 1:
            print(-1)
            exit()
        else:
            X[A[i]] = 1
if X[A[N - 1]] == 0:
    print(0)
else:
    if A[0] == A[N-1]:
        print(1)
    else:
        print(-1)