grades c#
using System;
class MainClass {
public static void Main (string[] args) {
int i = int.Parse(Console.ReadLine());
if(i>0 & i<100){
if(i>=90){
Console.WriteLine ("A");
}
if(i>=80 & i<90){
Console.WriteLine ("B");
}
if(i>=70 & i<80){
Console.WriteLine ("C");
}
if(i>=60 & i<70){
Console.WriteLine ("D");
}
if(i<60){
Console.WriteLine ("F");
}
}
}
}