Factorial Program in C using Recursion

/*
Write a program in C for factorial program in c using recursion
*/

#include<stdio.h>

long factorial(int);

main()
{
int num;
long f;

printf("ENTER A NUMBER TO FIND FACTORIAL :");
scanf("%d",&num);

if(num<0)
printf("NEGATIVE NUMBERS ARE NOT ALLOWED");
else
{
f = factorial(num);
printf("%d!=%ld",num,f);
}
return(0);
}

long factorial(int n)
{
if(n==0)
return(1);
else
return(n*factorial(n-1));
}