app.post() returns index of C:/ instead of text

I am new to web development(creating my first local server) and I can’t understand what I am doing wrong.

Following is my code on HTML:

<!DOCTYPE html>
<html lang="en" dir="ltr">
  <head>
    <meta charset="utf-8">
    <title>Calculator</title>
  </head>
  <body>
  <form  action= "/" method="post">
  <input type="text" name="num1" placeholder="First Number">
  <input type="text" name="num2" value="" placeholder="Second number">
  <button type="submit" name="submit">Calculate</button>
  </form>

  </body>
</html>

and JS :

var express = require("express");
var app = express();

app.get("/", function(req, res)
{
  res.sendFile(__dirname + "/index.html");
})

 app.post("/index.html", function(req,res){
     res.send("Thanks for posting that!");
 });
app.listen(3000, () => {
  console.log("Server is running on port 3000")
});

But the Output I am getting when I press (calculate button) is the (index of C:/) i.e the directory of C and all the files in it …

image of output i am getting after i press calculate

What I am using:

noje.js, html5 , nodemon , hyper terminal , atom text editor

What I want to achieve:

A server that sends a request to display an html form that accepts 2 numbers and then displays the sum as a text message when some1 presses the calculate(submit) button.

What I have tried:

  1. form action= “/index.html” method=”post”

Well, like u guessed, just refreshes the form

  1. app.post(“/”, function(req,res)

Same Output(index of C)

Sorry For asking noob questions .. but can you please explain to me what I am doing wrong and point me in the right direction.