#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int *ptr = mem;
for(int i=0;i<n-3;i++)
{
if (((*ptr) +1 == *(ptr +1)) && ((*ptr) +2 == *(ptr +2)))
{
return i;
}
ptr++;
}
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;
}