Recursion_natural number

#include<stdio.h>

int sum(int n);

int main()
{
    int n,concat;
    printf("Enter an positive integer: ");
    scanf("%d",&n);
    concat=sum(n);
    printf("Sum = %d",concat);
    return 0;
}
int sum(int n)
{
    if(n!=0)
     return n+sum(n-1);  
}