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