#include <stdio.h>
int find_pattern(int *mem, int n)
{
int *ptr = mem;
int index = 0;
while (ptr < mem + n - 2)
{
if (*(ptr + 1) - *ptr == 1 && *(ptr + 2) - *(ptr + 1) == 1)
{
return index;
}
ptr++;
index++;
}
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;
}