title: How do I sum numbers using a prompt, like a simple calculator?
body: <p>I tried to do a REALLY simple thing using JavaScript, a percentage calculator. This is the code:</p>
  
  <pre><code>var num = prompt("What is the number?")
  var perc = prompt("What is the percentage of change?")
  var math = num / (perc + 100) * 100
  var result = alert(eval(math))
  </code></pre>
  
  <p>But, for some reason, I can sum, for example:</p>
  
  <pre><code>var num1 = 15
  var num2 = 100
  alert(num1 + num2)
  </code></pre>
  
  <p>It will display 115, but I can't sum using something like this:</p>
  
  <pre><code>var num1 = prompt("Input a number.")
  var num2 = 100
  alert(num1 + num2)
  </code></pre>
  
  <p>If I write 15 in num1, the alert will display 15100. I tried some things, but none of them worked, so I really need help on this.</p>