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