I can't understand why is it an error?

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SoloLearn
{
class Program
{
static void Main(string[] args)
{
int x = 3;
Console.WriteLine(x);
int y = x++;
Console.WriteLine(y);
}
}