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