#include<iostream>
using namespace std;
int main (){
    int score;
    cin>>score;
    if(score<100 && score>90){
        cout<<"A";
    }
    else if(score>=80 && score<=89){
        cout<<"B";
    }
    else if(score>=70 && score<=79){
        cout<<"C";
    }
    else if(score>=60 && score<=69){
        cout<<"D";
    }
    else if(score<60){
        cout<<"F";
    }
    else (
        cout<<"Invalid input";
    }
    return 0;
}