var my_string = "";
    
    if(marks.length < 31){
        my_string = "FF";
    } else if(marks.length > 30 && marks.length < 41){
        my_string = "DD";
    }else if(marks.length > 40 && marks.length < 51){
        my_string = "CD";
    }else if(marks.length > 50 && marks.length < 61){
        my_string = "CC";
    }else if(marks.length > 60 && marks.length < 71){
        my_string = "BC";
    }else if(marks.length > 70 && marks.length < 81){
        my_string = "BB";
    }else if(marks.length > 80 && marks.length < 91){
        my_string = "AB";
    }else {
        my_string = "AA";
    }
    
    console.log(my_string);