import 'dart:io';
void main() {
// Print a message asking the user for input
print("Enter your name:");
// Use stdin.readLineSync() to read the user's input
String? name = stdin.readLineSync();
String s = name ?? "";
String grade="";
if(int.tryParse(s) != null) {
int num = int.parse(s);
switch(num) {
case >=10 && <=20:
grade = "F";
break;
case >=21 && <=50:
grade = "D";
break;
case >=51 && <=90:
grade = "C";
break;
}
print(grade);
}
else {
print("The input is not a number");
}
}