coding style

#include <iostream>
using namespace std;
int main(){
     cout<<"Hello world"<<endl;
     return 0;
}


OR

#include <iostream>
int main(){
    std::cout<<"Hello world"<<std::endl;
    return 0;
}

I've been told many times that the second option is better coding
practice, but I fail to understand why.  I mean, if one had to move to
another namespace in the program, then only then do you have to
change.  The first option, imo, makes the code more readable.
What is this forum's opinion?

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