//Do not declare variable marks.
//Write your code below this line.
var markStr;
if(marks <= 30) {
    markStr = 'FF';
}
else if(marks <= 40) {
    markStr = 'DD';
}
else if(marks <= 50) {
    markStr = 'CD';
}
else if(marks <= 60) {
    markStr = 'CC';
}
else if(marks <= 70) {
    markStr = 'BC';
}
else if(marks <= 80) {
    markStr = 'BB';
}
else if(marks <= 90) {
    markStr = 'AB';
}
else {
    markStr = 'AA';
}
console.log(markStr);