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