fn sum_str(a: &str, b: &str) -> Result<i64, Box<dyn std::error::Error>> {
  let a = a.to_string().parse::<i64>()?;
  let b = b.to_string().parse::<i64>()?;

  Ok(a + b)
}

fn main()
{
  let a = "a123";
  let b = "456";

  let sum = sum_str(a, b);
  if let Ok(s) = sum {
    println!("{} + {} = {}", a, b, s)
  } else {
    println!("Errore!")
  }
}