package main

import (
	"fmt"
	"time"
    "sync"
)

var wg sync.WaitGroup


func say(s string) {
	for i := 0; i < 5; i++ {
	  time.Sleep(100 * time.Millisecond)
	  fmt.Println(s)
	}
	wg.Done() // <=> wg.Add(-1)
}

func main() {
	wg.Add(2) // register two tasks.
	go say("world")
	say("hello")
	fmt.Println("re")
	wg.Wait()
}