Same formula working in JavaScript but not in C#

Question

This JavaScrpit function produce Correct Result

 

<script type="text/javascript">

function Function() {

amount = document.getElementById("Amount").value;

months = document.getElementById("Months").value;

intrest = document.getElementById("Intrest").value;

quaters = Math.floor(months / 3);

monthpayment = parseInt(amount) * ((Math.pow(intrest / 400 + 1, quaters) - 1) / (1-(Math.pow(intrest / 400 + 1,(-1/3)))));

document.getElementById("MaturityAmt").value = monthpayment.toFixed(2);

}

</script>

 

But Same logic not working in C#

 

void MyFunction()

{

double amount =Convert .ToDouble ( textBox1.Text) ;

double months =Convert .ToDouble ( textBox2.Text) ;

double intrest =Convert .ToDouble ( textBox3.Text) ;

double quaters = Math.Floor (months / 3);

double maturityamt = int.Parse (textBox1.Text ) * ((Math.Pow (intrest / 400 + 1, quaters) - 1) / (1 - (Math.Pow (intrest / 400 + 1, (-1 / 3)))));

textBox4.Text = maturityamt.ToString ();

}