#include <stdio.h>

void swap(int *a, int *b)
{
	int tmp;
	tmp = *a;
	*a = *b;
	*b = tmp;
}

int main(){

	int n, start, ne, old, x, y;
	while(1+scanf("%d", &n))
	{
		start = 1;
		scanf("%d %d", &x, &y);
		old = y-x;
		for(int i=2; i<n; i++)
		{
			scanf("%d", &x);
			swap(&x, &y);
			ne = y-x;
			if(ne != old)
			{
				start = i;
			}
			old = ne;
		}
		
		printf("%d\n", start);
	}
				
			

	return 0;
}