#include<iostream>
#include<string>
using namespace std;

class student {
    private:
         string name;
         string subject;
         int marks;
    public:
        student(string n, string s, int m){
            name = n;
            subject = s;
            marks = m;
        }
        void displayGrade() {
            if (name.empty() || subject.empty() || marks < 0 ||marks>100){
                cout << "Invalid input"<< endl;
                return 0;
            }
            cout<< "Name :" << name << endl;
            cout<< "Subject:"<< subject << endl;
            cout<< "Marks: "<< marks << endl;
            
            if (marks >= 90){
                cout << "Grade: A" << endl;
            }
            else if (marks >= 50) {
                cout << "Grade: C"<< endl;
            }
            else {
                cout<< "Grade: F" << endl;
        }
}