int n;
string n1;
float a,b=0;
Console.WriteLine("Nhap vao so n");
n1 = Console.ReadLine();
n = int.Parse(n1);
for (int e = 1;e<=n;e++)
{
b = 1;
for (int e1 = 1;e1<=e;e1++)
{
b = b*e1;
}
if (e1%2=0) b=b*-1;
a+=b;
}
Console.Write("Gia tri S = "+a);
Console.ReadLine();