#include<iostream>
using namespace std;

void printGrade(int score){
    if(score<0 || score>100){
        cout<<"Invalid input"<<endl;
    }
    else if(score>=90){
        cout<<"A"<<endl;
    }
    else if(score>=80){
        cout<<"B"<<endl;
    }
    else if(score>=70){
        cout<<"C"<<endl;
    }
    else if(score>=60){
        cout<<"D"<<endl;
    }
    else{
        cout<<"F"<<endl;
    }
}
int main(){
    int studentScore;
    cin>>studentScore;
    pointGrade(studentScore);
    return 0;
}