using System;
public class HelloWorld
{
public static void Main(string[] args)
{
int degree=int.Parse(Console.ReadLine());
if (degree<=100°ree>=85)
{
Console. WriteLine("A");
}
else if(degree<85°ree>=75){
Console.WriteLine ("B");
}
else if(degree<75°ree>=65){
Console.WriteLine ("C");
}
else if(degree<65°ree>=50)
{
Console.WriteLine("D");
}
else{
Console.WriteLine("F");
}
}
}
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