#include <bits/stdc++.h>

typedef long long ll;
typedef long double ld;

using namespace std;

#define rep(i, a, n) for (int i = (a); i < (n); i++)
#define per(i, a, n) for (int i = (n) - 1; i >= (a); i--)

int main(void) {
	ios::sync_with_stdio(false);
	int n;
	while(cin >> n) {
        vector<ll> a(n);
        rep(i,0,n) {
            cin >> a[i];
        }
        if (n == 1) {
            cout << 1 << endl;
            continue;
        }
        ll d = a[n-2] - a[n-1];
        int bad = -1;
        per(i,0,n-1) {
            if (a[i] - a[i+1] != d) {
                bad = i;
                break;
            }
        }
        if (bad == -1) {
            cout << 1 << endl;
        } else {
            cout << (bad+2) << endl;
        }
    }
	return 0;
}