逐次実行の素数計算

PrimeNumber.cs

Console.Write("求めたい値を入力 >");
string target_str = Console.ReadLine();
long target = long.Parse(target_str);

System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
bool flg = false;
sw.Start();
long i = 2;
for (i = 2; i < target / i; i++)
{
    if(target % i == 0)
    {
        flg = true;
        break;
    }
}
sw.Stop();
if (flg)
    {
        Console.WriteLine(target + "は素数ではない");
        Console.WriteLine("処理時間" + sw.ElapsedMilliseconds + "ms");
    }else
    {
        Console.WriteLine(target + "は素数");
        Console.WriteLine("処理時間" + sw.ElapsedMilliseconds + "ms");
    }