2.3

int main()
{
    int n;
	printf("nhap n tu ban phim: ");
	scanf("%d", &n);
	//Neu i > 5 thi se roi vao truong hop 1 o tren
	if(n > 5){
	    //Neu no la so chan thi tinh tiep, khong thi stop
	    if(n % 2 != 0)
	        return 0;
	    int result = 0;
	    //tinh i cho den khi nao i <= 2 thi stop
	    for(int i = 1; i <= n; i++) {
	        result = giaithua(i) / (i+2);
	    }
	    printf("S: %d", result);
	    return 0;
	}
	/*
	* Con neu 2 < n < 6 va la so le thi tinh tiep
	* khong thi ko print ra cai gi het
	*/
    if(n > 2 && n % 2 != 0)
	    printf("S: %d", giaithua(n+2));
	return 0;
}

int giaithua(int a) {
    if(a <= 1)
        return a;
    int total = 1;
    for(int i = 2; i <= a; i++) {
        total *= i;
    }
    return total;
}