Rust Grade Calculator

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
}