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