#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define int long long
typedef long double ld;

int n;
pair<int, int> a[200005];

main() {
	ios_base::sync_with_stdio(0), cin.tie(0);
	cin >> n;
	for (int i = 1; i <= n; ++i) {cin >> a[i].first, a[i].second = i;
	}
	sort(a + 1, a + n + 1);
	if (n == 2) {
		cout << 2 << '\n';
		return 0;
	}
	if (n == 3) {
		cout << 3 << '\n';
		return 0;
	}
	int d = a[2].first - a[1].first, lst = a[2].first, cnt = 0;
	bool f = 1;
	for (int i = 3; i <= n; ++i) {
		if (a[i].first != lst + d) {
			if (cnt) {
				f = 0;
				break;
			}
			cnt = a[i].second;
		} else lst = a[i].first;
	}
	if (f) {
		cout << (cnt == 0 ? 1 : cnt) << '\n';
		return 0;
	}
	d = a[3].first - a[1].first, lst = a[3].first, cnt = a[2].second;
	f = 1;
	for (int i = 4; i <= n; ++i) {
		if (a[i].first != lst + d) {
			if (cnt) {
				f = 0;
				break;
			}
			assert(false);
			cnt = a[i].second;
		} else lst = a[i].first;
	}
	if (f) {
		cout << cnt << '\n';
		return 0;
	}
	d = a[3].first - a[2].first, lst = a[3].first, cnt = a[1].second;
	f = 1;
	for (int i = 4; i <= n; ++i) {
		if (a[i].first != lst + d) {
			if (cnt) {
				f = 0;
				break;
			}
			assert(false);
			cnt = a[i].second;
		} else lst = a[i].first;
	}
	if (f) {
		cout << cnt << '\n';
		return 0;
	}
	cout << -1;
	return 0;
}