#include <stdio.h>
#include <stdlib.h>


int main(){
	int current = 0;
	int N;
	int *n;
	int i;
	while(scanf("%d", &N) > 0){
		n = (int*)malloc(N*sizeof(int));
		for(i = 0; i < N; i++){
			scanf("%d", n + i);
		}
		int i = 0;
		current = 0;
		while(i < N-2){
			if(n[i+1] - n[i] != n[i+2] - n[i+1]){
				current = i + 1;
			}
			i++;
		}
		printf("%d\n", current + 1);
			
	}
	return 0;
}