#include<stdio.h>

int doSum(int);
int main()
{
    int num,sum;
    printf("Enter a Number to perform Sum : ");
    scanf("%d",&num);

    sum = doSum(num);

    printf("Sum of Digits of Given Number is:  %d",sum);
    return 0;
}

int doSum(int num)
{

    static int sum =0,r;

    if(num!=0)
    {
        r=num%10;
        sum=sum+r;
        doSum(num/10);
    }

    return sum;
}

 OUTPUT:

Enter a Number to perform Sum : 258
Sum of Digits of Given Number is:  15