#include<stdio.h>
#include<string.h>
#define max 100
int main(){
    int q;
    scanf("%d",&q);
    if(q<0){
        printf("Invalid input");
        return 0;
    }
    int stack[max];
    int top=-1;
    char cmd[10];
    int value;
    for(int i=0;i<q;i++){
        scanf("%s",cmd);
        if(strcmp(cmd,"PUSH")==0){
            scanf("%d",&value);
            if(top<max-1){
                stack[++top]=value;
            }
        }else if(strcmp(cmd,"POP")==0){
            if(top==-1){
                printf(" Stack Underflow");
            }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){
                    printf("Stack Underflow\n");
                }else{
                   for(int j+0;j<=top;j++){
                       printf("%d ",stack[j]);
                   }
                   printf("\n");
                }
            }
        }
    return 0;
}