After entering a number in a textbox the code will display that number in the Fibonacci sequence in another textbox

namespace WindowsFormsApp1 { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void btnCalculate_Click(object sender, EventArgs e) { int N = int.Parse(txtNumber.Text); double d2 = 0; double d1 = 1; double fib = d1 + d2; for (int i = 3; i < N; i++) { d2 = d1; d1 = fib; fib = d1 + d2; } txtFibo.Text = fib.ToString(); } } }