how to sum of numbers using recursion

#include<iostream>

using namespace std;

int sum(int num)

{ 

         if(num==0)

         return 0;

         else return num+sum(num-1);

}

int main()

{

int n;

cout<<"Enter any number please"<<endl;

cin>>n;

cout<<"factorial is "<<sum(n)<<endl;

}