Problem calculating the sum of arithmetic progression when using prompt()

Question

I am trying this code to add a number starting from 1 to n using this mathematical equation:n = n(n+1) /2

var n = prompt("enter a number");

function adding(n) {
  let N = n * (n + 1) / 2;
  return N;
}
alert(adding(n));

but it's not working in prompt. It works like this:

function adding(n) {
  let N = n * (n + 1) / 2;
  return N;
}
console.log("answer", adding(100));