#include <iostream>
#include <string>

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

int main() {
    int n;
    cin>>n;
    printGrade(n);
    
    return 0;
}