#include <stdio.h>
#include <string.h>
int main(){
    int stack[100],top=-1;
    int q,x;
    char cmd[20];
    scanf("%d",&q);
    if(q<0){
        printf("{Invalid input");
        return 0;
    }
    while(q--){
        scanf("%s",cmd);
        if(strcmp(cmd,"PUSH")==0){
            scanf("%d",&x);
            stack[++top]=x;
        }
        else if(strcmp(cmd,"POP")==0){
            if(top==-1){
                printf("Stack under flow\n");
                else
                top--;
            }
            else if(strcmp(cmd,"PEEK")==0){
                if(top!=-1)
                printf("%d\n",stack[top]);
            }
            else if(strcmp(cmd,"DISPLAY")==0){
                for(int i=0;i<=top;i++)
                printf("%d",stack[i]);
                printf("\n");
            }
        }
        return 0;
}