#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int *p1 = mem;
int *p2 = (mem + 1);
int *p3 = (mem + 2);
for(int i; i<n; i++){
if((*p1+1 == *p2) && *(p2)+1==*p3)
return i;
*p1++;
*p2++;
*p3++;
}
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;
}