#include <iostream>
#include <future>
#include <chrono>
#include <vector>

using namespace std;

int square(int x) {
    return x * x;
}

int main() {

  int sum = 0;
  int v = 0;

  vector<future<int>> a;

  for (int i = 1; i <= 20; i++) {
     a.push_back(async(&square, i));
  }

  for (auto & avs : a ) {
    v = avs.get();
    sum += v;
  }

    cout << "The thread returned " << sum << endl;
    return 0;
}