You must put them in an array - and since you know how many elements you will have in the array - the calculation should be easy! You should only parse the numerical value.
Here it is:
using System;
class SumOfFiveNumbers
{
static void Main()
{
string[] numbers = Console.ReadLine().Split(' ');
decimal a = decimal.Parse(numbers[0]);
decimal b = decimal.Parse(numbers[1]);
decimal c = decimal.Parse(numbers[2]);
decimal d = decimal.Parse(numbers[3]);
decimal e = decimal.Parse(numbers[4]);
Console.WriteLine(a+b+c+d+e);
}
}