#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 command[10];
    int value;
    for(int i=0;i<q;i++){
        scanf("%s",command);
        if(strcmp(command,"PUSH")==0){
            scanf("%d",&value);
            if(top<MAX-1){
                stack[++top]=value;
        }
    }
    else if(strcmp(command,"POP")==0){
        if(top==-1){
            printf("stack underflow\n");
        }
        else{
            top--;
        }
    }
    else if(strcmp(command,"PEEK")==0){
        if(top==-1){
            printf("stack underflow\n");
        }
        else{
            printf("%d\n",stack[top]);
        }
    }
    else if(strcmp(command,"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;
}