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