//Do not declare variable marks.
//Write your code below this line.

function processData(marks) {
    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");
    //Enter your code here
} 

process.stdin.resume();
process.stdin.setEncoding("ascii");
_input = "";
process.stdin.on("data", function (marks) {
    _input += input;
});

process.stdin.on("end", function () {
   processData(_input);
});