#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"
template<class T> void chmax(T &a, const T b){ a = max(a, b); }
template<class T> void chmin(T &a, const T b){ a = min(a, b); }

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    ll n;
    cin >> n;
    vector<ll> a(n);
    rep(i, n) cin >> a[i];
    ll bv = -1;
    vector<ll> da;
    rep(i, n) {
        if (bv == a[i]) continue;
        da.push_back(a[i]);
        bv = a[i];
    }
    vector<ll> idx(n + 1, -1);
    ll ans = 0;
    rep(i, sz(da)) {
        if (idx[da[i]] == -1) {
            idx[da[i]] = i;
        }
        else {
            if ((idx[da[i]] == 0) && (i == (sz(da) - 1))) {
                ans = 1;
            }
            else {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    cout << ans << endl;
    return 0;
}