rgument 2: cannot convert from 'int' to 'System.Func<int, decimal>'

int lineReading;

if (inputStudentFile == "1")
{
    inputStudentFile = "studenten-1.txt";
    using (FileStream file = new FileStream(inputStudentFile, FileMode.Open, FileAccess.Read))
    {
        using (StreamReader reader = new StreamReader(file))
        {
            while (!reader.EndOfStream)
            {
                lineReading =         Convert.ToInt32(reader.ReadLine());
                nums.Add(lineReading);
                Console.WriteLine(lineReading);
            }
            if (inputMenu == "1")
            {
                nums.Sum(lineReading);
            }
        }
    }
}

int lineReading;

if (inputStudentFile == "1")
{
    inputStudentFile = "studenten-1.txt";
    using (FileStream file = new FileStream(inputStudentFile, FileMode.Open, FileAccess.Read))
    {
        using (StreamReader reader = new StreamReader(file))
        {
            while (!reader.EndOfStream)
            {
                lineReading =         Convert.ToInt32(reader.ReadLine());
                nums.Add(lineReading);
                Console.WriteLine(lineReading);
            }
            if (inputMenu == "1")
            {
                nums.Sum(lineReading);
            }
        }
    }
}