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