C语言相关程序

#include<iostream.h>
long f(int n)
{
	if(n<0)
	{
		cout<<"negative argument to fact!"<<endl;
		return(-1);
	}
		else if (n<=1)
			return(1);
		else return (n*f(n-1));
	
}
void main()
{
	long f(int n);
	int n;
	cout<<"please input n:"<<endl;
	cin>>n;
	cout<<"n!="<<f(n)<<endl;
}