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