'''
from math import gcd
from itertools import accumulate, combinations, permutations, product
from bisect import bisect, bisect_left
from heapq import heappush, heappop
from functools import reduce
from operator import add, mul, sub, truediv, floordiv, mod

from decimal import *
getcontext().prec = 500

import string
ascii = string.ascii_lowercase
'''
from collections import deque
from sys import stdin


def main():
    input = lambda: stdin.readline()[:-1]
    N = int(input())
    A = tuple(map(int, input().split()))

    a1 = [A[0]]
    for i in range(1, N):
        if a1[-1] != A[i]:
            a1.append(A[i])

    if len(a1) == 1:
        print(0)
        return

    a2 = deque(a1)
    for _ in [0] * len(a1):
        n = a2.popleft()
        if n != a2[-1]:
            a2.append(n)

    if len(set(a1)) != len(a2):
        print(-1)
    else:
        print(len(a1) - len(a2))


main()