#include <iostream>
#include <condition_variable>
#include <mutex>
#include <thread>
#include <iostream>
#include <queue>

std::queue<int> items_;
std::condition_variable cv;
std::mutex mutex_;

void Push(int item) {
  std::unique_lock<std::mutex> lock(mutex_);

  items_.push(item);
  if (items_.size() == 1) {
    cv.notify_all();
  }
}

void Pop(int& item) {
  std::unique_lock<std::mutex> lock(mutex_);

  while (items_.empty()) {
    cv.wait(lock);
  }
  item = items_.front();
  items_.pop();
}

void Process() {
  int el;
  std::cin >> el;
  Push(el);
  Pop(el);
  std::cout << el;
}

int main() {
  std::thread t1 = std::thread(Process);
  std::thread t2 = std::thread(Push);
}