How to fix the Cannot POST /index.html error?

The below is the example of calculator app with HTML and Javascript.

When I execute the program with nodemon, access the localhost:3000, and push the submit button, the error is shown on the browser(Google Chrome).

[nodemon] starting `node calculator.js`
Server started on port 3000.

Error Message

There is no error when executing the program, so how I can find the fixing points?

Cannot POST /index.html



<!DOCTYPE html>
<html lang="en" dir="ltr">
    <meta charset="UTF-8">
    <form action="/" method="post">
        <input type="text" name="n1" placeholder="First Number">
        <input type="text" name="n2" placeholder="Second Number">
        <button type="submit" name="submit">Calculate</button>


const express = require("express");
const bodyParser = require("body-parser");

const app = express();
app.use(bodyParser.urlencoded({extended: true})); //store the numbers in the forms

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

    var num1 = Number(req.body.n1);
    var num2 = Number(req.body.n2);
    console.log(num1, num2); //no results shown

    var result = num1 + num2;
    res.send("The result of the calculation is " + result);

app.listen(3000, function(){
    console.log("Server started on port 3000.");