#include <cstdio>
#include <climits>
#include <cmath>
#include <iostream>
#include <iomanip>
#include <string>
#include <cstdio>
#include <climits>
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <utility>
#include <queue>
#include <cstring>
#include <set>
#include <map>
#include <complex>

#define rep(i, n) for (int i = 0; i < int(n); i++)
using namespace std;
long long MOD = 1000000007;
long long INF = 1000000000000000; //10^15
typedef long long ll;
typedef unsigned long long ull;


ll powMod(ll x, ll n, ll mod) {
  if (n == 0) return 1;
  ll t = powMod(x, n/2, mod);
  t = t * t % mod;
  if (n & 1) return t * x % mod;
  return t;
}

ll gcd(ll a, ll b) {
  if (a == 0 || b == 0) return a + b;
  if (b > a) return gcd(b, a);
  return gcd(b, a % b);
}




int main(void) {

  int n, a[100100];
  cin >> n;
  rep(i, n) {
    cin >> a[i];
    a[i]--;
  }


  vector< vector<int> > v(n + 10);

  vector<int> b;
  int k = a[0];
  int j = 0;
  while (a[j] == a[0]) j++;
  for (int i = j; i < n; i++) b.push_back(a[i]);

  if (j == n) {
    cout << 0 << endl;
    return 0;
  }

  rep(i, b.size()) v[b[i]].push_back(i);

  bool f = true;
  rep(i, n) {
    if (v[i].size() > 1) {
      rep(j, v[i].size()-1) {
	if (v[i][j+1]-v[i][j] > 1) f = false;
	if (!f) break;
      }
    }
    if (!f) break;
  }

  if (!f) cout << -1 << endl;
  else {
    if (a[0] == a[n-1]) cout << 1 << endl;
    else cout << 0 << endl;
  }

  return 0;

}