#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main(){
    long long N;
    cin >> N;
    vector<long long> a(N);
    for(int i = 0; i < N; i++){
        cin >>  a[i];
        a[i]--;
    }
    vector<int> zi;
    zi.push_back(a[0]);
    for(int i = 1; i < N; i++){
        if(a[i] != a[i - 1]) zi.push_back(a[i]);
    }
    vector<int> cnt(N, 0);
    for(int i = 0; i < zi.size(); i++){
        cnt[zi[i]]++;
        if(cnt[zi[i]] > 2){
            cout << -1 << endl;
            return 0;
        }
        if(cnt[zi[i]] == 2 && i == zi.size() - 1){
            cout << 1 << endl;
            return 0;
        }
        else if(cnt[zi[i]] == 2){
            cout << -1 << endl;
            return 0;
        }
    }
    cout << 0 << endl;
}