#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep(i,n) for(ll i=0;i<(n);i++)
#define pll pair<ll,ll>
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))

using namespace std;

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;}

const ll INF=1e9+7;

int main()
{
    ll n;
    cin >> n;
    vector<ll> a(n);
    vector<ll> s;
    ll cnt=0;
    rep(i,n){
        cin >> a[i];
        
    }
    rep(i,n){
        if(i==0){
            continue;
        }
        if(a[i]!=a[i-1]){
            s.push_back(a[i-1]);
        }
    }
    s.push_back(a[n-1]);
    ll m=s.size();
    map<ll,bool> ans1;
    map<ll,bool> ans2;
    rep(i,m-1){
        if(ans1.count(s[i])){
            cout << -1 << endl;
            return 0;
        }
        else{
            ans1[s[i]]=true;
        }
    }
    rep(i,m-1){
        if(ans2.count(s[i+1])){
            cout << -1 << endl;
            return 0;
        }
        else{
            ans2[s[i+1]]=true;
        }
    }
    if(s.size()>=3 && s[0]==s[m-1]){
        cout << 1 << endl;
        return 0;
    }
    else{
        cout << 0 << endl;
        return 0;
    }
    return 0;
}