input()
A=input().split()
#A=list(map(int,input().split()))
index=[ [] for x in range(len(A)) ]
index={"dummy":[]}
for i in range(len(A)):
    index[A[i]] = index[A[i]]+[i] if A[i] in index.keys() else [i]

gaps=0

for k in index.keys():
    prev=None
    for j in index[k]:
        if prev!=None:
            if j-prev != 1:
                gaps=gaps+1
        prev=j
    if gaps>1:
        print(-1)
        exit()

if gaps >1:
    print(-1)
elif gaps==1 and A[0] != A[-1]:
    print(-1)
elif gaps==1 and A[0] == A[-1]:
    print(1)
elif gaps==0:
    print(0)