#include <stdio.h>
#include <stdint.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int8_t count = 1;
for (size_t i = 1; i < n; i++) {
if (((*(mem+i))-1) == *(mem+i-1)) {
count++;
}
else if (count > 1) {
count--;
}
if (count >= 3) 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;
}