#include<iostream>
using namespace std:
int main(){
    int i;
    cin>>i;
    if(i>90&&i<=100)
    {
        cout<<"A";
    }else if(i<89&&i>=80)
    {
        cout<<"B";
    }else if(i>=70&&i<79)
    {
        cout<<"C";
    }else if(i>=60&&i<=69)
    {
        cout<<"D";
    }else if(i>=59&&i<=0)
    {
        cout<<"F";
    }else if(i>=101)
    {
        cout<<"Invalid input"
    }
    return 0;
}