#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
if (n < 3) return -1;
int i = 0;
while (i < n - 2) {
int a = *(mem + i);
int b = *(mem + i + 1);
int c = *(mem + i + 2);
if (b == a + 1 && c == b + 1) {
return i;
}
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;
}