// Example program
#include <iostream>
#include <string>
#include <vector>
#include <exception>
#include <stdexcept>
template <typename T>
class Stack {
  public:

  T pop() {
    if (size() == 0) {
      throw std::runtime_error("Stack is empty");
    }
    auto result = data.back();
    data.erase(data.end() - 1);
    return result;
  }
  void push(const T& value) {
   data.emplace_back(value);
  }
  std::size_t size() const {
      return data.size();
  }

  private:
  std::vector<T> data;

};

int main()
{
    Stack<int> stack;
    std::cout << stack.size() << std::endl;
    stack.push(10);
    stack.push(20);
    try {
        std::cout << stack.size() << std::endl;
        std::cout << stack.pop() << std::endl;
        std::cout << stack.pop() << std::endl;
        std::cout << stack.pop() << std::endl;
    } catch (std::exception &e) {
        std::cout << e.what() << std::endl;
    }
}