#include <cmath>
#include <iostream>

namespace Math
{
  double const e = exp(1.0);
}

namespace Phys
{
  double const e = 1.609e-18; // Elektronenladung in As
}

int main()
{
  std::cout << "Euler-Zahl      e = " << Math::e << '\n'
            << "Elektronenmasse e = " << Phys::e << "As" << '\n';
  {
    using namespace Math;
    std::cout << "Euler-Zahl      e = " << e << '\n';
  }
  {
    using namespace Phys;
    std::cout << "Elektronenmasse e = " << e << "As" << '\n';
  }
}