rust 1.17.0
use std::io;

fn main() {
	
	println!("\n\tEnter your score: ");
	
	//read user input
	let mut input= String::new(); io::stdin() 	
		.read_line(&mut input) 
		.expect("failed to read input."); 
	//convert input to integer
	let score: i32 = input.trim().parse().expect("invalid input");
	
	let s= calc(score);
	
	print!("\n\tYour grade: {}",s);
	
	return ;
}

fn calc(sco: i32) -> &'static str{
	let mut gra= "F";
	
	match sco { 	
		90...100 => { gra= "A"; } 	
		80...89 => { gra= "B"; }
		70...79 => { gra= "C"; }
		60...69 => { gra= "D"; }
		_ => { println!("\n\tERROR: Invalid score\n"); }	
	}
	gra
}