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