Sum of digits

Question

I am trying to make this program to solve a homework problem the fun way but i got stuck 

 any help would be great, i just started a few days ago so dont make it to advanced

 

 

using System;

namespace TESTS
{
class Switch_test_1
{
static void Main(string[] args)
{
Int64 input = 10;
int output;
while (input < 10){
#find the sum of digits and apply it to output
int total = input - output;
print total;

}
}
}
}