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

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  
  int n;
  cin >> n;
  
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a.at(i);
  
  vector<int> x(n + 1);
  for (int i = 1; i < n; i++) {
    if (a.at(i) != a.at(i - 1)) x.at(a.at(i))++;
  }
  
  int kind = 0;
  bool flag = true;
  for (int i = 1; i <= n; i++) {
    if (x.at(i) > 0) kind++;
    if (x.at(i) > 1) flag = false;
  }
  
  if (flag) {
    if (a.front() == a.back() && kind > 0) cout << "1\n";
    else cout << "0\n";
  }
  else {
    cout << "-1\n";
  }
}