#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int consecutive_len = 1;
for (int i = 1; i < n; i++){
mem++;
if (*(mem - 1) == (*mem) - 1){
consecutive_len++;
if (consecutive_len == 3){
return i-2;
}
}
else{
consecutive_len = 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;
}