How to force JS to do math instead of putting two strings together

I need javascript to add 5 to an integer variable, but instead it treats the variable as a string, so it write out the variable, then add 5 onto the end of the "string". How can I force it to do math instead?

var dots = document.getElementById("txt").value; // 5
function increase(){
    dots = dots + 5;
}

Output: 55

How can I force it to output 10?