Stack ADT: PUSH / POP
Implement an integer stack and process commands to feel LIFO.
Commands: PUSH x (push, no output); POP (pop and print the top, or EMPTY if empty).
Input: line 1 command count n; then n command lines.
Output: one line per POP.
Example: PUSH 3 / PUSH 7 / POP / PUSH 1 / POP → 7 then 1.