Multithreaded Summation: Using fetch_add Method with Relaxed Semantic

// synchronisationWithFetchAddRelaxed.cpp
...
void sumUp(std::atomic<unsigned long long>& sum, const std::vector<int>& val, 
           unsigned long long beg, unsigned long long end){
    for (auto it = beg; it < end; ++it){
        sum.fetch_add(val[it], std::memory_order_relaxed);
    }
}
...

// synchronisationWithFetchAddRelaxed.cpp
#include <chrono>
#include <iostream>
#include <mutex>
#include <random>
#include <thread>
#include <utility>
#include <vector>
#include <atomic>
constexpr long long size = 100000000;   
constexpr long long fir =  25000000;
constexpr long long sec =  50000000;
constexpr long long thi =  75000000;
constexpr long long fou = 100000000;
std::mutex myMutex;
std::atomic<unsigned long long> sum = {};
void sumUp(std::atomic<unsigned long long>& sum, const std::vector<int>& val, 
           unsigned long long beg, unsigned long long end){
    for (auto it = beg; it < end; ++it){
        sum.fetch_add(val[it], std::memory_order_relaxed);
    }
}
int main(){
  std::cout << std::endl;
  std::vector<int> randValues;
  randValues.reserve(size);
  std::mt19937 engine;
  std::uniform_int_distribution<> uniformDist(1,10);
  for (long long i = 0 ; i < size ; ++i) 
      randValues.push_back(uniformDist(engine));
 
  
  const auto sta = std::chrono::steady_clock::now();
  
  std::thread t1(sumUp, std::ref(sum), std::ref(randValues), 0, fir);
  std::thread t2(sumUp, std::ref(sum), std::ref(randValues), fir, sec);
  std::thread t3(sumUp, std::ref(sum), std::ref(randValues), sec, thi);
  std::thread t4(sumUp, std::ref(sum), std::ref(randValues), thi, fou);   
  
  t1.join();
  t2.join();
  t3.join();
  t4.join();
  
  std::chrono::duration<double> dur= std::chrono::steady_clock::now() - sta;
  std::cout << "Time for addition " << dur.count() 
            << " seconds" << std::endl;
  std::cout << "Result: " << sum << std::endl;
  std::cout << std::endl;
}