n=int(input())
if n==1:
    print(0)
    exit()
A=list(map(int,input().split()))
pre=-1
kouho=set([])
out=[]
for i in range(n):
    if A[i]==pre:
        continue
    if A[i] in kouho:
        out.append(A[i])
    kouho.add(A[i])
    pre=A[i]
if not out:
    if len(out)==1 and A[0]==A[-1]:
        print(1)
    else:
        print(-1)
else:
    print(0)