#include <iostream>
using namespace std;
int main() {
int grade ;
cin>>grade ;
if (grade>=90) {
cout<<"A\n";
}
else if (grade>=80) {
cout<<"B\n";
}
else if (grade>=70) {
cout<<"C\n";
}
else if (grade>=60) {
cout<<"D\n";
}
else {
cout<<"F\n";
}
return 0;
}
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