public static int[] GetTotalSum(int[] sum,int target)
    {
        for (int i = 0; i < sum.Length; i++)
        {
            for (int j = i+1; j < sum.Length; j++)
            {
                if (sum[i]+sum[j]==target)
                {
                    return new[] {i, j};
                }
            }
        }

        return null;
    }