// editor4
#include<stdio.h>
#include<stdlib.h>
typedef struct node{
    char data[20];
    struct node*next;
}Node;
Node*head=NULL,*tail;
void create(char*ch){
    Node*newNode=(Node*)malloc(sizeof(Node));
    strcpy(newNode->data,ch);
    newNode->next=NULL;
    if(head==NULL){
        head=newNode;
        tail=newNode;
    }
    else{
        tail->next=newNode;
        newNode->prev==newNode;
        tail=newNode;
    }
}
int main(){
    int size,i;
    scanf("%d",&size);
    if(size<0){
        printf("Invalid input");
        return 0;
    }
}