Why mix output logic with determination of x and y?

Why use loops to build string (or Stringbuilder)? Obfuscates code.

Here is my solution:-

  static void decentNumber(int n)
{
    int x = 0;
    int y = 0;
    for (int i = n; i >= 0; i--)
    {
        if (i % 3 == 0 && (n - i) % 5 == 0)
        {
            x = i;
            y = n - i;
            break;
        }
    }

    var res = y == 0  && x==0? "-1" : new string('5', x) + new string('3', y);
    Console.WriteLine(res);
}