#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int *p=mem;
for(int i=0;i<n-3;i++,p++){
if(*(p+1)==*p+1 && *(p+2)==*p+2){
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;
}