Count pairs in a sorted array whose sum is less than x

//first approach is brute force
int pairs_count(int arr[],int n,int k){
    int l = 0, r = n-1,res= 0;
    while (l < r){
        if (arr[l] + arr[r] < k){    // If current left and current right have sum smaller than x,
            res += (r - l);          // the all elements from l+1 to r form a pair with current l.
            l++;
        }
        else    // Move to smaller value
            r--;
    }
    return res;
}