/**
 *  author: Huan 
 *  躺平日记
 *  created: 2024-03-31 12:14:08
**/
#include <bits/stdc++.h>
#define ll long long
using namespace std;

const int mod = 1e9 + 7;
const int N = 1e6 + 10;

void solve()
{
    int n;
    cin >> n;
    vector<int>v(n);
    set<int>se;

    int idx;
    for(int i = 0 ;i < n ;i ++)
    {   
         cin >> v[i];
         if (v[i] == 0){
            idx = (i + 1) % n;
         }
         se.insert(v[i]);
    }

    // if (se.size() == 1 && v[0] == 0){
    //     cout << 0 << endl;
    //     return ;
    // }
    if (se.size() == 1){
        cout << n + 1 << endl;
        return ;
    }
    if (se.size() != n){
        cout << -1 << endl;
        return ;
    }

    int sum = v[idx];

    if (idx != 1){
        sum ++ ;
    }
    
    cout << sum << endl;


}

signed main(void) {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    int tt = 1; //cin >> tt;
    while(tt -- ) solve();
    return 0;
}