title: How to make an input only take numbers
body: <p>I havn't been able to find a solution to this problem on the internet (maybe im not looking hard enough) but i can't figure out how to make an input only take numbers. Im trying to get the input to go through some equations and the program brakes everytime i put a letter in the input. I was wondering if there was a way to detect if the input was a letter or number. I'll show my program.</p>
  
  <pre><code>    Radius=input("What is the radius of the circle/sphere?")
  
      Areacircle=(int(Radius)**2)*3.14159265359
      Perimetercircle=2*3.14159265359*int(Radius)
      Permsphere=4*3.14159265359*(int(Radius)**2)
      Areasphere=(4/3)*3.14159265359*(int(Radius)**3)
  
      print("The radius' length was:",Radius)
      print("The surface area of each circle is:",Areacircle)
      print("The perimeter of the circle is:",Perimetercircle)
      print("The volume of the sphere would be:",Areasphere)
      print("The perimeter of the Sphere would be:",Permsphere)
  </code></pre>