input()
A=list(map(int,input().split()))

index=[]
prev=A[0]
for i in range(len(A)):
    if prev !=A[i]:
        if A[i] in index:
            print(-1)
            exit()
        else:
            prev=A[i]
            index.append(A[i])

if A[0] != A[-1]:
    print(0)
else:
    print(1)
exit()