#include<stdio.h>
#include<string.h>
int main(){
    int q,st[100],top=-1,x;
    char cmd[10];
    scanf("%d",&q);
    while(q--){
        scanf("%s",cmd);
        if(!strcmp(cmd,"PUSH")){
            scanf("%d",&x);
            st[++top]=x;
        }
        else if(!strcmp(cmd,"POP")){
            if(top<0)
            printf("Stack Underflow\n");
            else top--;
        }
        else if(!strcmp(cmd,"PEEK")){
            if(top<0)
            printf("Stack Underflow\n");
            else
            printf("%d\n",st[top]);
        }
        else if(!strcmp(cmd,"DISPLAY")){
            if(top<0)
            printf("Stack Underflow\n");
            else for(int i=0;i<=top;i--)
            printf("\n");
        }
    }
    return 0;
}