var result="";
if(marks>90)
    result="AA";
else if(marks>80 && marks<=90)
       result="AB";
     else if(marks>70 && marks<=80)
        result="BB";
        else if(marks>60 && marks<=70)
        result="BC";
          else if(marks>50 && marks<=60)
        result="CC";
           else if(marks>40 && marks<=50)
        result="CD";
           else if(marks>30 && marks<=40)
        result="DD";
           else if(marks<=30)
               result="FF";
console.log(result);