#include <stdio.h>
#include <math.h>

int arr[1001];
int main() {
	int N;
	while (scanf("%d\n", &N) == 1) {
		int i;
		for (i=1; i<=N; i++) {
			scanf("%d ", arr+i);
		}
		int diff;
		if (N>1) {
			diff = arr[N]-arr[N-1];
			for (i=N-1; i>=2; i--) {
				if (arr[i]-arr[i-1] != diff) break;
			}
			printf("%d\n", i);
		} else {
			printf("%d\n", 1);
		}
	}

	return 0;
}