n = int(input())
A = list(map(int,input().split()))
l = []
for a in A:
    if l and l[-1] == a:
        continue
    l.append(a)

dic = {}
for i in l:
    dic[i] = dic.get(i,0)+1

num = 0
for k,v in dic.items():
    if v == 1:
        continue
    if v > 2:
        print(-1)
        exit()
    if l[0] == l[-1] == k:
        num += 1
    else:
        print(-1)
        exit()
print(num)