#include<iostream>
using namespace std;
int main(){
    int score;
    of(!(cin>>score)){
        cout<<"invalid input\n";
        return 0;
    }
    if(score<0||score>100){
        cout<<"invalid input\n";
    }
    elseif(score>=90){
        cout<<"A\n";
    }elseif(score>=80){
        cout<<"B\n";
    }elseif(score>=70){
        cout<<"C\n";
    }elseif(score>=60){
        cout<<"D\n";
    }else{
        cout<<"F\n";
    }
    return 0;
}