Lvalue required....?

#include

int main()

{

   enum{i=10,j=20,k=50};

   printf("%d\n",++k);

 

   return 0; 

}

...guys what is the output, and give your opinion why it is so...? What exactly is the concept of Lvalue...and in what circumstances it happens so...? Leave as far as possible examples in your comments.