#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int conceq_len_count =0;
for (int needle_pos = 0; needle_pos < n; needle_pos++)
{
if( ((*(mem + needle_pos + 1)) - (*(mem + needle_pos))) == 1)
{
conceq_len_count++;
if (conceq_len_count == 2)
{
return (needle_pos - 1);
}
}
}
return -1;
}
int main() {
int n, arr[100];
scanf("%d", &n);
for (int i = 0; i < n; i++) {
scanf("%d", &arr[i]);
}
int res = find_pattern(arr, n);
printf("%d", res);
return 0;
}