import java.util.Scanner;

public class Amusement {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(sc.hasNextInt()){
            int nums = sc.nextInt();
            int[] numbers = new int[nums];

            for(int i = 0; i < nums; i++){
                numbers[i] = sc.nextInt();
            }

            int diff = numbers[nums - 2] - numbers[nums - 1];
            int lastIndex = nums - 1;
            for(int i = nums - 2; i > 0; i--){
                int currDiff = numbers[i-1] - numbers[i];

                if (diff != currDiff) {
                    break;
                }
                lastIndex = i;
            }

            System.out.println(lastIndex);
        }
    }
}