static bool LaSoNguyenTo(int a)
for (int i = 2; i <= Math.Sqrt(a); i++)
static void Main(string[] args)
Console.WriteLine("Nhap N");
N = int.Parse(Console.ReadLine());
Console.WriteLine($"{i}");
Console.WriteLine($"TBC cac so nguyen to nho hon N la: {Tong}/{CacSoNguyenTo}");