4

#include <iostream>

namespace first
{
int a=10;
int b=20;

namespace second
{
double a=1.02;
double b=5.002;
void hello();
}

void second::hello()
{
std::cout <<"hello world"<<std::endl;
}
}

int main()
{
using namespace first;

std::cout<<second::a<<std::endl;
second::hello();
}