#include <stdio.h>
int find_pattern(int *men, int n) {
for(int i=0;i<n-3;i++){
if(*(men+i)+1==*(men+i+1) && *(men+i)+2==*(men+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;
}