#include <bits/stdc++.h>

#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("O3")

#define F first
#define S second
#define pb push_back
#define llong  long long
#define int llong
#define ld long double
#define endl '\n'

#ifdef BUCKY_IS_ALIVE
#define cerr if(0) cerr
#else
#endif // BUCKY_IS_ALIVE

using namespace std;

const int N = 1e5 + 5;
const int M = 1e5 + 5;
const int B = 2100;
const int sum = 2e9;
const int MOD = 1e9 + 7;
const int MOD1 = 1e9 + 7;
const int rx[4] = {0, 1, -1, 0};
const int ry[4] = {1, 0, 0, -1};
const char rr[5]= {'5', 'N', 'E', 'S', 'W'};



main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    srand(time(0));
#ifdef BUCKY_IS_ALIVE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    int n;
    cin >> n;
    vector <pair <int, int> > all;
    for(int i = 0; i < n; i++) {
        int x;
        cin >> x;
        all.pb({x, i});
    }

    sort(all.begin(), all.end());
    int cur = all[1].F - all[0].F;
    for(int i = 1; i < n; i++) {
        if (all[i].F - all[i - 1].F == cur)
            continue;
        else {
            vector <int> a;
            for(int j = 0; j < n; j++) {
                if (i == j)
                    continue;
                a.pb(all[j].F);
            }

            bool ok = 1;

            for(int j = 1; j < a.size(); j++) {
                if (a[j] - a[j - 1] == cur)
                    continue;
                ok = 0;
                break;
            }

            if (ok) {
                cout << all[i].S + 1 << endl;
                return 0;
            }
            break;

        }
    }

    if (all.size() == 2) {
        cout << all[0].S + 1 ;
        return 0;
    }

    int nom = all[0].S;
    reverse(all.begin(), all.end());
    all.pop_back();
    reverse(all.begin(), all.end());

    bool ok = 1;

    cur = all[1].F - all[0].F;
    for(int i = 1; i < all.size(); i++) {
        if (all[i].F - all[i - 1].F == cur)
            continue;
        ok = 0;
        break;
    }

    if (ok){
        cout << nom + 1;
        return 0;
    }

    cout << -1;









        return 0;


    }