#include <iostream>
using namespace std;

class Student {
public:
    int A;
    int B;
    int C;

    void gasan() {
        int gap = (A + B + C) / 3;
        if(gap >= 90){
            cout << "A" <<endl;
        }
        else if(gap >= 80){
            cout << "B" <<endl;
        }
        else if(gap >= 70){
            cout << "C" <<endl;
        }
        else cout << "F" << endl;
    }
};

int main() {
    Student s;
    cin >> s.A >> s.B >> s.C;
    s.gasan();
    
}

//QRBQB,Q9_Q99Q,ZqZ9,N9GU,1F5FA,AekE,24h,AGB