#include <stdio.h>

void swap(int *a, int *b)
{
	int tmp;
	tmp = *a;
	*a = *b;
	*b = tmp;
}
int arr[1000];
int i;
int main(){

	int n, start, d, x, y;
	while(1+scanf("%d", &n))
	{
		for(i=0; i<n; i++)
		{
			scanf("%d", &arr[i]);
		}
		d = arr[n-1] - arr[n-2];

		for(i = n-2; i>=0;i--)
		{
			
			if(arr[i]-arr[i-1] != d)
				break;
			
		}
		if(i < 0) 
			i = 0;
		printf("%d\n", i+1);
	}
				
			

	return 0;
}