import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class Main {

    public static void main(String[] args) throws IOException {

        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));


        while (true)
        {
            String tmp = in.readLine();
            if(tmp == null) break;
            int num = Integer.parseInt(tmp);
            ArrayList<Integer> numbers = new ArrayList<>();
            StringTokenizer stringTokenizer = new StringTokenizer(in.readLine());

            for (int i = 0; i < num; i++)
            {
                numbers.add(Integer.parseInt(stringTokenizer.nextToken()));
            }
            boolean azDokonca = true;
            if(numbers.size() == 1 || numbers.size() == 2) System.out.println(1);
            else {
                int diff = 0;
                diff = numbers.get(numbers.size() - 1) - numbers.get(numbers.size() - 2);
                for(int i = numbers.size() - 2; i >= 1; i--)
                {
                    if(diff != numbers.get(i) - numbers.get(i - 1))
                    {
                        azDokonca = false;
                        System.out.println(i+1);
                        break;
                    }
                }
                if(azDokonca)
                {
                    System.out.println(1);
                }
            }

        }



    }
}