#include<bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i) 
#define rrep(i, a, b) for (int i = (a); i < (b); ++i)
#define PI acos(-1)
#define pcnt __builtin_popcountll
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
#define v(T) vector<T>
#define vv(T) v(v(T))
#define fi first
#define se second

using namespace std;
using ll = long long;
using ld = long double;
using P = pair<int, int>;
using LP = pair<ll, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;

template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) {if (a > b) { a = b; return true; } return false; }

ll INF = 1001001001;
ll LINF = 1001001001001001001ll;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll n; cin>>n;
    vl as(n); cin>>as;

    vl ps(n + 1, -1);
    bool f = false;
    rep(i, n) {
        if (as[i] == as[0]) {
            if (f) {
                if (i < n - 1 && as[i] != as[i + 1]) {
                    cout<<-1<<endl;
                    return 0;
                }
            }
            else if (i < n - 1 && as[i] != as[i + 1]) f = true;
            continue;
        }
        if (ps[as[i]] != -1) {
            if (ps[as[i]] == i - 1) ps[as[i]] = i;
            else {
                cout<<-1<<endl;
                return 0;
            }
        }
        else ps[as[i]] = i;
    }
    if (f && as[0] == as.back()) cout<<1<<endl;
    else cout<<0<<endl;
}