The rule of The Big Three - class

Question

Iam confused with the below question I did the program as per my understanding but it crashes what am I doing wrong? If someone can please assist me it would be much appreciated.
my main.cpp looks like this:
#include <iostream>
#include <iomanip>
#include "Number.h"
using namespace std;
int main()
{
Number n1(10);
Number n2 = n1;
n2.printNum();
n2.addOne();
n1 = n2;
n1.printNum();
return 0;
}
Then my header file looks like this:
#include <iostream>
using namespace std;
class Number
{
int *p;
public:
Number(int);
void addOne();
void printNum();
};
And the below parts for the constructor I need to complete there where it shows comments that's the part I should complete:
#include <iostream>
#include "Number.h"
using namespace std;
Number::Number(int a1)
{
*p = a1;//write the code needed to initialise the value of the member variable with a1
}
void Number::printNum()
{
cout << "The number is " << *p << endl;
}
void Number::addOne()
{
*p++;//write the code needed to increment the value of the member variable by one.
}
Then the question asks the below what should I do to the code to use the BIG THREE?
Consider the following program. Complete the class definition (where you are asked to) and check the output. You can see that that program works without error once it is completed. However, experts suggest that in any class that uses pointers and the new operator it is better to follow the rule of The Big Three. Modify the class definition to follow the rule of The Big Three and submit the new program and the output. Demonstrate the use of this pointer.
Thank you
Rohan