NAMESPACE ERROR '... not a namespace-name'

I started learning c++ and have namespace exercise I have some problem with namspaces

These are three files:

dane3.h

using namespace std;
namespace xddd
{
   const int QUARTERS = 4;
   struct Sales
   {
    double sales[QUARTERS];
    double average;
    double Max;
    double Min;
   };

   void setSales(Sales & s, const double ar[], int n);
   void showSales(const Sales & s);
}

dane2.cpp

using namespace std;
#include <iostream>

#include "dane3.h"
namespace xddd
{
void setSales(Sales & s, const double ar[], int n)
{
int point1 = ar[0];
int point2 = ar[0];
int point3 = 0;

for(int i=1;i < n;i++){
    if(point1 > ar[i])
        point1 = ar[1];}

for(int i=1;i < n;i++){
    if(point2 < ar[i])
        point2 = ar[1];}

for(int i=0;i < n;i++)
    point3 += ar[i];

s.average = point3;
s.Max = point2;
s.Min = point1;
}

void showSales(const Sales & s)
{
cout << s.average << "\n";
cout << s.Max << "\n";
cout << s.Min;
}
}

main.cpp

#include <iostream>
using namespace std;
#include "dane3.h"
using namespace xddd;
int main()
{
    Sales a;
    double table[4] = {10,20,30,40};
    setSales(a,table,QUARTERS);
    showSales(a);
}

I get the error:

 'xddd' is not a namespace-name

I think that error is very simple, but i dont know how to figure it BTW sorry for my english if I made some mistakes :D