Cannot access public static variable member

I've declared a public static member to keep the total count of the instances of my class. The code is as follows:

class Hello {
public:
    static int myCount;
    void test(){
        //do nothing
    };
    Hello(){
        Hello::myCount += 1;
    };
    ~Hello() {
        Hello::myCount -= 1;
    }
};

int main(int argc, const char * argv[]) {
    // insert code here...
    Hello *p1 = new Hello();p1->test();
    Hello *p2 = new Hello();p2->test();
    cout << Hello::myCount;

    return 0;
}

However, I when compile, it says:

Undefined symbols for architecture x86_64:
  "Hello::myCount", referenced from:
      _main in main.o
      Hello::Hello() in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I don't know where I'm wrong. It's been years from the last time I worked with c++, so could you please suggest a solution? Thank you.