#include <iostream>
#include <vector>
#include <thread>
#include <atomic>
using namespace std;

mutex sum_mutex;

void square(int x, int &sum) {
  int temp = x * x;
  sum_mutex.lock();
  sum += temp;
  sum_mutex.unlock();

  //sum += x * x;
}

int main() {

  int sum = 0;


    vector<thread> allthread;
    for (int i = 1; i <= 20; i++) {
        allthread.push_back(thread(&square, i, ref(sum)));
    }

    for (auto& th : allthread) {
        th.join();
    }
    cout << "sum = " << sum << endl;
    return 0;
}