# coding: utf-8
# Your code here!
import sys
read = sys.stdin.read
readline = sys.stdin.readline 

n,*a = list(map(int,read().split()))

res = [0]*(n+3)
now = 10**6

two = 0
for i in a:
    if i != now:
        res[i] += 1        
        if res[i]==2: two += 1
        elif res[i] == 3: two = 100
        now = i

if two >= 2:
    print(-1)
elif two == 1:
    if a[0] == a[-1] and res[a[0]] == 2:
        print(1)
    else:
        print(-1)
elif two==0:
    print(0)

#print(res[:5],two)