int bsearch(const int *arr, int n, int key) {
    int lo = 0, hi = n - 1;
    while (lo <= hi) {
        int mid = (lo + hi) / 2;
        if (arr[mid] == key) return mid;
        if (arr[mid] < key) lo = mid + 1;
        else hi = mid - 1;
    }
    return -1;
}