fcb030_unique

//#include<bits/stdc++.h>
#include<iostream>
#include<fstream>
#include<vector>
#include<algorithm>

#define long long long
#define nln '\n'

const long N = 2*1e5+10;

using namespace std;

// Global variables: f1, f2, n, a

fstream f1, f2;

inline void openf()
{
    f1.open("unique.inp", ios:: in);
    f2.open("unique.out", ios:: out);
}

inline void closef()
{
    f1.close();
    f2.close();
}

long n;
vector<pair<long, long>> a;

void data()
{
    f1.tie(0)->sync_with_stdio(0);
    f2.tie(0)->sync_with_stdio(0);
    cin.tie(0)->sync_with_stdio(0);
    cin >> n;
    for (long i = 0; i != n; ++i)
    {
        long x;
        cin >> x;
        a.push_back({x, i+1});
    }
}

long ans = -1;

void process()
{
    sort(a.begin(), a.end());

    //for (const auto &i : a)
    //  cout << i.second << " - " << i.first << nln;

    if (n == 1)
    {
        ans = 1;
        return;
    }

    if (a[0].first != a[1].first)
    {
        ans = a[0].second;
        return;
    }

    for (long i = 1; i != n-1; ++i)
    {
        if (a[i].first != a[i-1].first && a[i].first != a[i+1].first)
        {
            ans = a[i].second;
            return;
        }
    }

    if (a[n-1].first != a[n-2].first)
    {
        ans = a[n-1].second;
        return;
    }
}

void view()
{
    cout << ans << nln;
}

int main()
{
    openf();
    data();
    process();
    view();
    closef();
    return 0;
}