Reading a String that has integers, a number and its power and pushing them into a Stack

This is the code:

     #include <iostream>
     #include <string>

    using namespace std; 

    class Node {
        public:
          int data; 
          Node* next; 

          public: Node(int d) {
            data=d;
            next=NULL;
          }; 
          }; 


class Stack { 
      Node *head; 

      public: Stack() { 
      head = NULL; 
      }; 

   ~Stack(){
   while (head)
   {
      Node * temp = head;
      head = head->next;
      delete temp;
   }
}

      void push(int data); 
      int pop(); 
      bool isEmpty();
      void print();
      }; 

      void Stack::push(int data) 
      {
        Node * temp = new Node(data);
        temp->next=head;
        head=temp;
      }

      int Stack::pop() 
      { 
      if(head!=NULL){
        int x=head->data;
        Node * temp = head;
        head=head->next;
        delete temp;
        return x;
      }else{
        cout << "The stack is empty!";
        return -1;
      }

      }

      bool Stack::isEmpty(){
         return head==NULL;  
     } 

     void Stack::print(){
        Node * temp = head;
        while(temp!=NULL){
            cout << temp->data << " ";
            temp=temp->next;
         }
     }


int main()
{ 

    Stack St1, St2;
    char c;
    string Str;
    int i, a, length = 0;
    int n, p;
    bool flag = true;

    cout << "Enter the String, use this form: np (n is the number and p is the power)\n";
    getline(cin, Str);

    for(i=0; Str[i]!='\0'; i++)
    {
        c=Str[i];

        if('0'<=c && c<='9')
        {
            St1.push(c);
            length++;
        }
    }
    if(length%2 != 0)
    a = St1.pop();  /* To make sure the String that was entered has an even length
    (i.e. a number and its power) */

    if(length == 0)
    cout << "Your string didn't include any numbers!\n";

    while(!St1.isEmpty()&&flag)
    {
        p = St1.pop();
        n = St1.pop();

        if(p>0)
          St2.push(n);
        else
          flag = false;
    }

    if(flag)
      St2.print();

    cin >> a;
    return 0;

}

In main, when I enter a String of 2352, it always prints "Your string didn't include any numbers!". So obviously, nothing is being pushed into the stack. This program should reads a String that belongs to this family: {a^p b^p | p>0} based on the stack data structure. p is the power.. so if p > 0 it displays the values of a and b.. So in this case " 2352 " a = 2, p = 3, b = 5, and p =2.. since p > 0.. 2 and 5 should be printed. also, as long as i'm pushing integers only between 0 and 9 into the stack, that means p will never be negative.