public static int divisibleSumPairs(int n, int k, List<int> ar)
    {
        int count = 0;
        int[] remainders = new int[k]; 
        
        for (int i = 0; i < n; i++)
        {
            int currentRemainder = ar[i] % k;
            if (currentRemainder < 0) currentRemainder += k;
            int complement = (k - currentRemainder) % k; 
            count += remainders[complement];
            remainders[currentRemainder]++;
        }
        
        return count;
    }