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

int main() {
    int n, *x, i, diff, t;
    while (1) {
        if (scanf("%d", &n) != 1) break;
        x = malloc(sizeof(int)*n);
        for (i = 0; i < n; i++) {
            scanf("%d", (x + i));
        }
        if (n == 1) {
            printf("1\n");
            continue;
        }
        t = 1;
        diff = *(x + n - 1) - *(x + n - 2);
        for (i = n - 3; i >= 0; i--) {
            if ((*(x + i + 1) - *(x + i)) != diff) {
                printf("%d\n", i + 2);
                t = 0;
                break;
            }
        }
        if (t)
            printf("1\n");
    }
    return 0;
}