use std::sync::{Arc, Mutex};
use std::thread;

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = vec![];

    for _ in 0..4 {
        let c = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            let mut n = c.lock().unwrap();
            *n += 1;
        }));
    }

    for h in handles {
        h.join().unwrap();
    }
    println!("{}", *counter.lock().unwrap());
}