#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using vin=vector<int>;
using vll=vector<long long>;
using vvin=vector<vector<int>>;
using vvll=vector<vector<long long>>;
using vstr=vector<string>;
using vvstr=vector<vector<string>>;
using vch=vector<char>;
using vvch=vector<vector<char>>;
using vbo=vector<bool>;
using vvbo=vector<vector<bool>>;
using vpii=vector<pair<int,int>>;
using pqsin=priority_queue<int,vector<int>,greater<int>>;
#define mp make_pair
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep2(i,s,n) for(int i=(s);i<(int)(n);i++)
#define all(v) v.begin(),v.end()
#define decp(n) cout<<fixed<<setprecision((int)n)
const int inf=1e9+7;
const ll INF=1e18;

int main(){
    int n;cin>>n;
    vin a(n);
    rep(i,n)cin>>a[i];
    vbo j(n+1,true);
    j[a[0]]=false;
    int prev=0;
    if(a[0]!=a[n-1]){
        rep2(i,1,n){
            if(a[i]==a[i-1]||j[a[i]])j[a[i]]=false;
            else{
                cout<<-1<<endl;
                return 0;
            }
        }
        cout<<0<<endl;
    }
    else{
        int l=0;
        int r=n-1;
        rep2(i,1,n){
            if(a[i]!=a[i-1])break;
            l++;
        }
        if(l==n-1){
            cout<<0<<endl;
            return 0;
        }
        for(int i=n-2;i>=0;i--){
            if(a[i]!=a[i+1])break;
            r--;
        }
        rep2(i,l+1,r){
            if(a[i]==a[i-1]||j[a[i]])j[a[i]]=false;
            else{
                cout<<-1<<endl;
                return 0;
            }
        }
        cout<<1<<endl;
    }
}