#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
using ll = long long;
int main() {
  int n;
  cin >> n;
  vector<int> a(n), b(n + 1, 0);
  for(int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for(int i = 0; i < n - 1; i++) {
    if(a[i] == a[i + 1]) {
      b[i + 1] = 1;
    }
  }
  vector<int> c;
  for(int i = 0; i < n; i++) {
    if(b[i] == 0) {
      c.push_back(a[i]);
    }
  }
  map<ll, ll> mp;
  for(int i = 0; i < c.size(); i++) {
    mp[c[i]]++;
    if(mp[c[i]] >= 3) {
      cout << -1 << endl;
      return 0;
    } else if(mp[c[i]] == 2 && i == c.size() - 1 && c[i] == c[0]) {
      cout << 1 << endl;
      return 0;
    } else if(mp[c[i]] == 2) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << 0 << endl;
  return 0;
}