#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> used(N, 0);
    used[a[0]] = 1;
    int cnt = 1;
    for(int i = 1; i < N; i++){
        if(a[i] != a[i - 1] && used[a[i]] == 1 && i != N - 1){
            cout << -1 << endl;
            return 0;
        }
        else if(a[i] != a[i - 1] && used[a[i]] == 0) {
            cnt++;
            used[a[i]] = 1;
        }
    }
    if(cnt == N){
        cout << 0 << endl;
        return 0;
    }
    else{
        cout << 1 << endl;
        return 0;
    }
}