C :

#include <stdio.h>
 
#define BASE 10
 
int main(void)
{
    int n, digitsum;
 
    scanf("%d", &n);
 
    digitsum = 0;
    while(n) {
        digitsum += n % BASE;
        n /= BASE;
    }
 
    printf("%d\n", digitsum);
 
    return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
int shu;
int he=0;
int main()
{
	
	cin>>shu;
	while(shu>0)
	{
		he=he+shu%10;
		shu=shu/10;
	}
	cout<<he;
	
	return 0;
}