newbie :P

This is the task: Given two positive integers m and n. Determine the number m n last digit sum. Sharing the figure should be quantified.

Code:

#include <iostream>
using namespace std;
int main ()
{
    int   m,n,k,j=0,sum2=0,sum,sum1=0,z;


    cout<<"ievadiet skaitli M: \n";
    cin>>m;
    
      cout<<"ievadiet skaitli N: \n";
    cin >>n;


   for (int  i=0; i<=n; i++)
   {
      sum=m%10;
      
          sum1=sum1+sum;
       
do
      {      
m=m-10;
j++;     
 m=j;
      }
      while (m>=10);
         
//m=m/10;

      
}     

  cout<<sum1<<endl;

    
    system("pause");
    return 0;
}

Why this program is not the same with it :

Code:

#include <iostream>
using namespace std;
int main ()
{
    int   m,n,k,j=0,sum2=0,sum,sum1=0;


    cout<<"ievadiet skaitli M: \n";
    cin>>m;
    
      cout<<"ievadiet skaitli N: \n";
    cin >>n;


   for (int  i=0; i<=n; i++)
    {
    sum=m%10;
      
      sum1=sum1+sum;
       
m=m/10;
   
}     

       cout<<sum1<<endl;
 
    system("pause");
    return 0;
}