help with a foreach loop condition

Calculate the mean average value and assign that to the average variable.

 

using System;

namespace HelloWorld
{
class Program
{
static void Main(string[] args)
{
int sum = 0;
int[] marks = { 24, 10, 26, 20 };

foreach (int mark in marks)
{
int sum1 = marks.Sum();

}

Console.WriteLine(sum);
}
}
}