#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 Overflow\n");
            }
            else
            {
                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<=0;j++) 
                printf("%d", stack[j]);
                if(j != top)
                printf(" ");
            }
            printf("\n");
        }
    }
    else
    {
        printf("Invalid input\n");
        return 0;
    }
    return 0;
}