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 if (marks <= 30){
    console.log("FF");
}