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