请帮忙给出源代码

算阶乘的代码自己已经写出
#include<stdio.h>
unsigned long mul(int n);
int main()
{
int m;
puts("Calculate n! n=?\n");
scanf("%d", &m);
printf("%d!=%ld\n", m, mul(m));
getch();
return 0;
}
unsigned long mul(int n)
{
unsigned long p;
if(n>1)
p=n*mul(n-1);
else
p=1L;
return(p);
}