/* -*- coding: utf-8 -*-
 *
 * 1053.cc:  No.1053 ゲーミング棒 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

/* global variables */

int as[MAX_N], used[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  for (int i = 0; i < n; i++) scanf("%d", as + i), as[i]--;
  n = unique(as, as + n) - as;

  int c = 0;
  for (int i = 0; i < n; i++)
    if (++used[as[i]] == 2) c++;

  printf("%d\n", (c > 1) ? -1 : c);
  return 0;
}