#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
ll mod = 1000000007;

ll r(ll x, ll y) {
  if (y == 0) return 1;
  else if (y % 2 == 0) return r(x, y/2) * r(x, y/2) % mod;
  else return x * r(x, (y-1)/2) % mod * r(x, (y-1)/2) % mod;
}

int main() {
  ll n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  vector<int> b(1);
  b[0] = a[0];
  for (int i = 1; i < n; i++) {
    if (b.back() != a[i]) b.push_back(a[i]);
  }
  int m = b.size();
  bool FF = 1;
  if (b[0] == b[m-1]) {
    m--;
    FF = 0;
  }
  int now = 10000000;
  bool F = 1;
  vector<bool> f(n, 0);
  for (int i = 0; i < m; i++) {
    if (now != b[i]) {
      now = b[i];
      if (f[now]) {
        F = 0;
        break;
      } 
      f[now] = 1;
    }
  }
  if (F&&!FF && m != 0) cout << 1 <<endl;
  else if (F&&!FF && m == 0) cout << 0 <<endl;
  else if (F&&FF) cout << 0 <<endl;
  else cout << -1 <<endl;
}