#include <iostream>

using namespace std;

int main()
{
    std::ios_base::sync_with_stdio(false);


    int pocet;
    int *cisla;
    int startIndex;
    int d;


    while(true)
    {
        cin >> pocet;

        if(cin.eof())
            break;

        cisla = new int[pocet];

        for(int i = 0; i < pocet; i++)
        {
            cin >> cisla[i];
        }

        startIndex = pocet - 2;
        int d = cisla[pocet - 1] - cisla[pocet - 2];
        for(int i = pocet - 3; i >= 0; i--)
        {
            if(cisla[i + 1] - cisla[i] == d)
            {
                startIndex = i;
            }
            else
            {
                break;
            }
        }

        cout << startIndex + 1 << endl;

        delete[] cisla;
    }

    /*int pocet;
    int a1;
    int d;
    int startIndex = 1;
    int prev;
    int curr;

    while(true)
    {
        cin >> pocet;

        if(cin.eof())
            break;

        if(pocet != 0 && pocet != 1 && pocet != 2)
        {
            cin >> prev;
            cin >> curr;
            a1 = prev;
            d = curr - prev;
            startIndex = 1;
            for(int i = 2; i < pocet; i++)
            {
                prev = curr;
                cin >> curr;
                if(curr != prev + d)
                {
                    a1 = prev;
                    d = curr - prev;
                    startIndex = i;
                }
            }
        }

        cout << startIndex << endl;
    }*/


    return 0;
}