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