#pragma region
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std; typedef long double ld; typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define VPII vector<PII>
#define VPLL vector<PLL>
#define ALL(x) (x).begin(), (x).end()
constexpr int INF=1<<30; constexpr ll LINF=1LL<<60; constexpr ll mod=1e9+7; constexpr int NIL = -1;
template<class T>inline bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
#pragma endregion
//-------------------
const int MX = 1e5+5;
int a[MX];

int main(){
    cin.tie(0); ios::sync_with_stdio(false); //cout << fixed << setprecision(15);
    int n; cin >> n;
    FOR(i,1,n) cin >> a[i];

    vector<int> mp(MX, 0);
    int piv = INF;
    FOR(i,1,n) {
        if(mp[a[i]] == 0 or mp[a[i]]==i-1) {
            mp[a[i]] = i;
        } else {
            piv = i;
            break;
        }
    }
    // cout << piv << endl;

    if(piv == INF) {
        cout << 0 << endl;
        return 0;
    }

    FOR(i,piv,n) {
        if(a[1] != a[i]) {
            cout << -1 << endl;
            return 0;
        }
    }

    cout << 1 << endl;

    return 0;
}