import java.util.*;
public class Main 
{
	public static void main(String[] args) 
	{
		int a,b,n;
		String k;
		Stack<Integer> s = new Stack<Integer>();
		Scanner cin=new Scanner(System.in);
		while(cin.hasNext()) 
		{
			n=cin.nextInt();
			if(n==0)
				break;
			for(int i=0;i<n;i++)
			{
				k=cin.next();
				if(k.charAt(0)=='P')
				{
					s.push(cin.nextInt());
				}
				else if(k.charAt(0)=='O')
				{
					if(s.empty()!=true)
						s.pop();
				}
				else if(k.charAt(0)=='A')
				{
					if(s.empty()!=true)
						System.out.println(s.peek());
					else
						System.out.println("E");
				}
			}
			while(s.empty()!=true)
				s.pop();
			System.out.print("\n");
		}
		cin.close();
	}
}
/**************************************************************
	Problem: 2192
	User: admin
	Language: Java
	Result: Accepted
	Time:1005 ms
	Memory:55044 kb
****************************************************************/