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