#include <iostream.h>
using namespace std;
int main()
const unsigned int n1 = 20; //read only can't be changed
cout << "define and declaring constant \
read only variables : data types\n ";
cout << "constant data type = " << n1 <<endl;
return 0;

Use define or Costantant ?
Using constant data type has an advantage for programmers over #define preprocessor, because the constant data type  is understood and processed by the compiler, not just substitution of some text in the preprocessor. The error messages from compiler are easier to debug.