#include <bits/stdc++.h>
#define REP(i, m, n) for(int i = (int)(m); i < (int)(n); ++i)
#define rep(i, n) REP(i, 0, n)
using namespace std;
using ll = long long;
using ld = long double;
template<class T, class U> inline bool chmax(T &a, const U &b) { if(a < b) { a = b; return true; } return false; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if(a > b) { a = b; return true; } return false; }

int n, a[100000];

int main() {
  cin.tie(0); ios::sync_with_stdio(false);
  cin >> n;
  rep(i, n) cin >> a[i], a[i]--;
  int p = -1;
  vector<int> cnt(n, 0);
  rep(i, n) {
    if(a[i] != p) cnt[a[i]]++;
    p = a[i];
  }
  int over = 0;
  bool ok = true;
  rep(i, n) {
    if(cnt[i] == 0) continue;
    if(cnt[i] > 2) ok = false;
    else if(over == 0 && cnt[i] == 2) {
      over++;
    }
    else if(over > 0 && cnt[i] == 2) {
      ok = false;
    }
  }
  if(ok) {
    cout << over << '\n';
  }
  else {
    cout << -1 << '\n';
  }
  return 0;
}