#include <iostream>
using namespace std;
float daythua(int x);
void main()
{
	int n;
	cout <<"cout nhap so n: "; cin >> n; system("cls");
	cout <<" ket qua: "<< daythua(n)<<endl;
	system("pause");
}


float daythua(int x)
{
	int y=1;
	for (int i=0; i<x; i=i+1)
		y=y*(y-i);
	return ((1/y));
}