class Solution
{
public:
    void push(int node) 
    {
        stack1.push(node);
    }

    int pop() 
    {
        while(!stack1.empty())
        {
            stack2.push(stack1.top());
            stack1.pop();
        }
        if(!stack2.empty())
        {
            int nRet = stack2.top();
            stack2.pop();
            while(!stack2.empty())
            {
                stack1.push(stack2.top());
                stack2.pop();
            }

            return nRet;
        }
        return -1;
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};