#include <bits/stdc++.h>
using namespace std;
 
#define REP(i, s) for (int i = 0; i < s; ++i)
#define ALL(v) (v).begin(), (v).end()
#define COUT(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define EACH(i, s) for (__typeof__((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define DEBUG
#define int long long
#define INF 1e18
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T1, class T2> ostream& operator << (ostream &s, pair<T1,T2> P)
{ return s << '<' << P.first << ", " << P.second << '>'; }
template<class T> ostream& operator << (ostream &s, vector<T> P)
{ for (int i = 0; i < P.size(); ++i) { if (i > 0) { s << " "; } s << P[i]; } return s; }
template<class T> ostream& operator << (ostream &s, vector<vector<T> > P)
{ for (int i = 0; i < P.size(); ++i) { s << endl << P[i]; } return s << endl; }
template<class T> ostream& operator << (ostream &s, set<T> P)
{ EACH(it, P) { s << "<" << *it << "> "; } return s << endl; }
template<class T1, class T2> ostream& operator << (ostream &s, map<T1,T2> P)
{ EACH(it, P) { s << "<" << it->first << "->" << it->second << "> "; } return s << endl; }
template<class T>void show(vector<T>v){for (int i = 0; i < v.size(); i++){cerr<<v[i]<<" ";}cerr<<"\n";}
typedef long long ll;

signed main()
{
    int n;
    cin >> n;
    vector<int> a(n);
    REP(i,n){
        cin >> a[i];
    }
    map<int, int> mp;
    
    vector<int> b;
    b.push_back(a[0]);
    for (int i = 1; i < n; i++)
    {
        if(a[i] !=a[i-1]){
            b.push_back(a[i]);
        }
    }
    //show(b);
    bool flag = true;
    int idx = 0;
    //cerr << b.size() << endl;
    for (int i = 0; i < b.size(); i++)
    {
        //cerr << mp[b[i]];
        if (mp[b[i]] > 0)
        {
            flag = false;
            idx = i;
            break;
            
        }
        mp[b[i]]++;
    }
    if(flag){
        cout << 0 << endl;
    }
    else{
       // cerr << idx << endl;
        for (int i = idx; i < b.size(); i++)
        {
            if(b[i] !=b[0]){
                cout << -1 << endl;
                return 0;
            }
        }
        cout << 1 << endl;
    }

    return 0;
}