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


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

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

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