#include<iostream>
using namespace std;
int main(){
    int scrore;
    cin>>score;
    if(score<0 || score>150){
        cout<<"Invalid input";
    }
    else if(score>=90 && score<=100){
        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>=0 && score<=59){
        cout<<"F";
    }
    return 0;
}