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