#include <iostream>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdlib.h>
#include <cmath>

int main()
{
	int N, i, diff, index, tmp;
	int *pole;

	while(scanf("%d", &N) > 0) {
		pole = (int *) malloc ((N+1) * sizeof(int));

		for(i = 1; i <= N; i++) {
			scanf("%d", &pole[i]);
		}

		diff = pole[N] - pole[N-1];
		index = N;
		for(i = N-1; i > 1; i--) {
			tmp = pole[i] - pole[i-1];
			//intf("%d %d \n", diff, tmp);
			if(tmp == diff) {
				
				index = i;
			} else {
				break;
			}
		}

		printf("%d\n", index-1);
		free(pole);
	}	

	return 0;
}