#include <stdio.h>

int main() {
	int n = 0;
	while (scanf("%d", &n) > 0) {
		int currD = -2100000000, currI = -2100000000, lastNum = -2100000000;
		for (int i = 0; i < n; i++) {
			int num = 0;
			scanf("%d", &num);

			if (currI == -1) currI = i;
			else {
				int d = num - lastNum;
				if (currD != d) {
					currD = d;
					currI = i;
				}
			}

			lastNum = num;
		}
		printf("%d\n", currI);
	}
	return 0;
}