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");
      }
    
    }
  
}
}