N=int(input())
A=list(map(int,input().split()))
B=[A[0]]
for i in range(1,N):
    if A[i]!=A[i-1]:
        B.append(A[i])
from collections import Counter
C=Counter(B)

if max(C.values())==1:
    print(0)
else:
    D=[c-1 for c in C.values()]
    if sum(D)>1:
        print(-1)
    else:
        if A[0]==A[-1]:
            print(1)
        else:
            print(-1)