#include <stdio.h>
int find_pattern(int *mem, int n)
{
/* int *ptr = mem;
while (ptr < mem + (n - 3))
{
int a = *ptr;
if ((*(ptr + 1) == (a + 1)) && (*(ptr + 2) == (a + 2)))
{
return (int)(ptr-mem);
}
ptr++;
}
*/
int *p1 = mem;
int *p2 = mem + 1;
int *p3 = mem + 2;
for (int i = 0; i < n - 3; i++)
{
if (*p1 + 1 == *p2 && *p2 + 1 == *p3)
{
return i;
}
p1++;
p2++;
p3++;
}
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;
}