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