#include "bits/stdc++.h"
using namespace std;
 
#define REP(i, n) for (ll i = 0; i < n; i++)
#define REPR(i, n) for (ll i = n; i >= 0; i--)
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define FORR(i, m, n) for (ll i = m; i >= n; i--)
#define REPO(i, n) for (ll i = 1; i <= n; i++)
#define ll long long
#define INF (ll)1 << 60
#define MINF (-1 * INF)
#define ALL(n) n.begin(), n.end()
#define MOD 1000000007
#define P pair<ll, ll>

bool solve(vector<ll> s){
    vector<ll> v, cnt(110000);
    ll n = s.size();
    REP(i, n){
        if (i == n - 1 or s[i] != s[i + 1]) v.push_back(s[i]);
    }
	REP(i, v.size()){
        cnt[v[i]]++;
        if (cnt[v[i]] >= 2) return false;
    }
    return true;
}
int main() {
    ll n;
    cin >> n;
    vector<ll> s(n);
    REP(i, n)
        cin >> s[i];
    if(solve(s)){
        cout << 0 << endl;
        return 0;
    }
	if(s[0] != s[n - 1]){
        cout << -1 << endl;
        return 0;
    }
    while (s[0] == s.back()) s.pop_back();
	if(solve(s)){
        cout << 1 << endl;
        return 0;
    }
	cout << -1 << endl;
}