var button;

var nums;

function setup(){

  input = createInput();

  input.position(20, 30);

  button = createButton("submit");

  button.position(160, 30);

  if(button === true){

   nums = input.value();

  print(sum(nums)); 

  }

}

​

function sum(a,b,c){

  return a+b+c;

}