n = int(input())
a = list(map(int, input().split()))

from collections import defaultdict

block_count = defaultdict(int)
prev = None
for num in a:
    if num != prev:
        block_count[num] += 1
        prev = num

first = {}
last = {}
for i in range(n):
    num = a[i]
    if num not in first:
        first[num] = i
    last[num] = i

possible = True
for c in block_count:
    bc = block_count[c]
    if bc == 1:
        continue
    elif bc == 2:
        if a[0] == c and a[-1] == c:
            continue
        else:
            possible = False
            break
    else:
        possible = False
        break

if not possible:
    print(-1)
else:
    valid = True
    for c in first:
        left = first[c]
        right = last[c]
        for i in range(left, right + 1):
            if a[i] != c:
                valid = False
                break
        if not valid:
            break
    if valid:
        print(0)
    else:
        print(1)