c# winform Math question

Question

ii have 3 text boxes I have data in 2 of them, I need the third box to look at the first 2 and if textbox1 is less than textbox 2 than textbox 3 will equal whats in textbox2. If textbox1 is greater than textbox2 it will equal textbox1.

 

This is what I have tried as you notice the third line is Equal how can I use < less than?

 

 private void txtMinLogic_TextChanged(object sender, EventArgs e) {
string a = txtHalfDayUsage.Text;
string b = tbGreaterThan.Text;
            if (a.Equals(b))
            {
                txtMinLogic.Text = "";
            }
            else
            {
                txtMinLogic.Text = txtHalfDayUsage.Text;

            }
}