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