#include <bits/stdc++.h>
using namespace std;

#ifdef LOCAL
#include "debug.h"
#else
#define DEBUG(...)
#endif

template <class S> auto rle(const S& s) {
  vector<decltype(make_pair(s[0], 1))> res;
  for (auto c : s)
    if (empty(res) or res.back().first != c) res.emplace_back(c, 1);
    else ++res.back().second;
  return res;
}

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<int> a(n);
  for (auto&& e : a) cin >> e;
  auto v = rle(a);
  set<int> se;
  int m = size(v);
  for (int t = 0; t < m; ++t) {
    if (se.count(v[t].first)) {
      if (t == m - 1 and v[0].first == v[t].first) {
        cout << "1\n";
        exit(0);
      } else {
        cout << "-1\n";
        exit(0);
      }
    }
    se.insert(v[t].first);
  }
  cout << "0\n";
}