duolingo.js

var lessons = document.getElementsByClassName("lessons-left"); // get the lessons
var tempLessons = []; // temporary array
for(i=0; i<lessons.length; i++) {
	tempLessons.push(lessons[i].innerText);//clean the html
}
lessons = tempLessons;//restore original array name
var s = 0; // initalize a var fo sum
for(i=0; i<lessons.length; i++) {
	if (lessons[i] !== '') {//catch if nulled value
		s+=parseInt(lessons[i][2]); 
	}
}