You have the line
dots = document.getElementById("txt").value;
in your file, this will set dots to be a string because the contents of txt is not restricted to a number.
to convert it to an int change the line to:
dots = parseInt(document.getElementById("txt").value, 10);
Note: The 10
here specifies decimal (base-10). Without this some browsers may not interpret the string correctly. See MDN: parseInt
.