#include<iostream>
using namespace std;
void printGrade(intscore){
    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 score;
    cin>>score;
    printGrade(score);
    return 0;
}