#include<stdio.h>
#include<string.h>
#define MAX 1000
int main(){
    int stack[MAX];
    int top = -1;
    int q,x;
    char command[10];
    scanf("%d", &q);
    for(int i=0;i<q;i++){
        scanf("%s",command);
        if(strcmp(command,"PUSH")==0){
            scanf("%d", &x);
            if(top<MAX-1){
                stack[++top]=x;
            }
        }
        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;
        
    }