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