#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin >> n;
    long a[n];
    for(int i=0;i<n;i++)cin >> a[i];
    vector<int>b;
    int ii=0;
    while(ii<n){
        int tmp = a[ii];
        b.push_back(tmp);
        while(a[ii] == tmp)ii++;
    }
    vector<bool>d(n+1,false);
    int ans = 0;
    for(int i=0;i<b.size();i++){
        if(!d[b[i]]){
            d[b[i]] = true;
        }else{
            if(!(i==b.size()-1 && b[i]==b[0])){
                ans = -1;
                break;
            }else{
                ans = 1;
            }
        }
    }
    cout << ans << endl;
    
    
}