Just put a condition on the content.
bool contLetter = false;
string reqNumber = textBox1.Text.Trim();
for (int i = 0; i < reqNumber.Length; i++)
{
if (!char.IsNumber(reqNumber[i]))
{
contLetter = true;
}
}
if (contLetter)
{
}
Then once you have confirmed a number, confirm it's value is between the chosen ranges.
(I leave that as an excercise for the OP)