#include <stdio.h>
#include <stdint.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
for(uint8_t i=0;i<n;i++)
{
int firstIndex=i;
int secondIndex=i+1;
int thirdIndex=i+2;
if((firstIndex<n) && (secondIndex<n) && (thirdIndex<n))
{
if(((*(mem+firstIndex)+1)==((*(mem+secondIndex)))) && ((*(mem+firstIndex)+2)==((*(mem+thirdIndex))))) return firstIndex;
}
else
{
break;
}
}
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;
}