#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;
constexpr int INF = 2e9;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i,n) cin >> a[i], --a[i];
  a.emplace_back(INF);
  int prev = 0;
  vector<int> p;
  for (int i = 0; i <= n; i++) {
    if (a[i] != a[prev]) {
      p.emplace_back(a[prev]);
      prev = i;
    }
  }
  vector<int> cnt(n, 0);
  bool ok = true;
  for (int i = 0; i < p.size(); i++) {
    cnt[p[i]]++;
    if (cnt[p[i]] > 2) ok = false;
  }
  int d = 0;
  for (int i = 0; i < n; i++) {
    if (cnt[i] == 2) {
      d = 1;
      if (p.back() != p.front()) ok = false;
    }
  }
  if (ok) cout << d << endl;
  else cout << -1 << endl;
  return 0;
}