#include <cstdint>
#include <iostream>
#include <numeric>
#include <string>
#include <thread>
#include <vector>

int main(int argc, char *argv[]) {
  if (argc != 3) {
    std::cerr << "Please, give the number of elements and number of threads in "
                 "the command line.\n";
    return 1;
  }

  int const N = std::stoi(argv[1]);
  if (N <= 0) {
    std::cerr << "Number of elements must be positive.\n";
    return 2;
  }

  int const num_threads = std::stoi(argv[2]);
  if (num_threads <= 0) {
    std::cerr << "Number of threads must be positive.\n";
    return 2;
  }

  // Create an array from 0 to N-1.
  std::vector<std::int64_t> array(N);
  std::iota(begin(array), end(array), 0);

  std::int64_t sum = 0;

  // A function to sum part of the array in the variable sum
  auto part_sum = [&sum, &array = std::as_const(array),
                     num_threads](int tid) {
    for (size_t j = tid; j < array.size(); j += num_threads) {
      sum += array[j];
    }
  };

  std::vector<std::thread> threads;
  for (int i = 0; i < num_threads; ++i) {
    threads.emplace_back(part_sum, i);
  }

  for (auto &thd : threads) {
    thd.join();
  }

  std::int64_t n64 = N;
  if (sum == n64 * (n64 - 1) / 2) {
    std::cout << "Nice, I can sum!" << std::endl;
  } else {
    std::cout << "Back to school!" << std::endl;
  }

  return 0;
}