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