#include <stdio.h>

int main()
{

  int number1;
  int number2;

  int *p; //the address of p is allocated, but the value of p is not defined

  printf("the value of null point %d\n", *p);

  number1=20;
  p=&number1;
  printf("the value of that p point to is %d\n", *p);


  number1=30;
  p=&number1;
  printf("the value of that p point to is %d\n", *p);

  return 0;
}