#include<stdio.h>
#include<string.h>

#define MAX_SIZE 100

int main() {
    int n;
    if (scanf("%d, &n") != 1) return 0;
    
    if (n < 0) {
        printf("Invalid input");
        return 0;
    }
    
    int stack[MAX_SIZE];
    int top - 1;
    
    for (int i = 0; i < n; i++) {
        char cmd[10];
        scanf("%s", cmd);
        
    if (strcmp(cmd,"PUSH") == 0) {
        int x;
        scanf("%d", &x);
        if (top < MAX_SIZE - 1) {
            stack[++top] =x;
        }
    }
    else if (strcmp(cmd, "POP") ==0){
        if (top == -1) {
            printf("stack Underflow\n")''
        }
    else {
        top--;
        }
    }
    else if (strcmp(cmd, "PEEK") ==0){
        if (top == -1) {
            printf("stack Underflow\n")''
        }
    else {
        printf("%d"\n, stack[top]);
        }
    }
    else if (strcmp(cmd, "DISPLAY") == 0) {
        if (top == -1) {
    }
    else{
        for (int j = 0; j <= top; j++) {
            printf("%d ".stack[j]);
        }
        printf("\n");
    }
  }
}