My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too. Or just jump to my GitHub repository.

       
#include <iostream>
       
#include <vector>
       
#include <cmath>
       
 
       
int main()
       
{
       
  unsigned long long limit = 1ULL << 50;
       
  std::cin >> limit;
       
 
       
  // largest prime factor that may appear as a square
       
  unsigned long long root  = (unsigned long long)sqrt(limit);
       
 
       
  // count number of distinct prime factors
       
  std::vector<unsigned char> numPrimeFactors(root, 0);
       
  // does any prime factor occurs multiple times ?
       
  std::vector<bool> ignore(root, false);
       
  for (unsigned long long prime = 2; prime < root; prime++)
       
  {
       
    // skip if not a prime number
       
    if (numPrimeFactors[prime] != 0)
       
      continue;
       
 
       
    // add the prime factor to all multiples
       
    for (auto j = prime; j < root; j += prime)
       
      numPrimeFactors[j]++;
       
 
       
    // all multiples have at least one prime factor multiples times, mark as invalid
       
    auto square = prime * prime;
       
    for (auto j = square; j < root; j += square)
       
      ignore[j] = true;
       
  }
       
 
       
  // count all numbers that are not squarefree
       
  unsigned long long notSquarefree = 0;
       
  for (unsigned long long base = 2; base < root; base++)
       
  {
       
    // at least one prime factor occurs multiple times ?
       
    if (ignore[base])
       
      continue;
       
 
       
    // all multiples are not squarefree
       
    auto square = base * base;
       
    auto numMultiples = limit / square;
       
 
       
    // if the number of prime factors is odd, then these multiples are new
       
    if (numPrimeFactors[base] % 2 == 1)
       
      notSquarefree += numMultiples;
       
    else // else: when even number of prime factors, then we have seen these numbers before
       
      notSquarefree -= numMultiples;
       
  }
       
 
       
  // display result
       
  auto result = limit - notSquarefree;
       
  std::cout << result << std::endl;
       
  return 0;
       
}

This solution contains 9 empty lines, 11 comments and 3 preprocessor commands.