Stack Push Pop Top

#include<stdio.h>
#include<stdlib.h>
typedef struct Data
{
    int a;
    struct Data *next,*prev;
}Data;
Data *head=NULL;

void push(int x)
{
    Data *new_node=(Data*)malloc(sizeof(Data));
    new_node->a=x;
    new_node->next=NULL;
    if(head==NULL)
    {
        head=new_node;
        printf("Push Value %d\n",head->a);
        return;
    }
    new_node->next=head;
    head=new_node;
    printf("Push Value %d\n",head->a);
}

int pop()
{
    Data *dlt=head;
    int x;
    if(head==NULL)
    {
        printf("Empty Stack!\n");
        return -1;
    }
    if(head->next==NULL)
    {
        head=NULL;
        x = dlt -> a;
        free(dlt);
        return x;
    }
    head=head->next;
    x = dlt -> a;
    free(dlt);
    return x;
}

void top()
{
    int x;
    if(head==NULL)
    {
        printf("Noting in Top!\n");
        return;
    }
    x=head->a;
    printf("Top Value: %d\n", x);
}

int main()
{
    pop();
    top();
    push(5);
    pop();
    top();
    push(7);
    push(6);
    push(5);
    top();


    return 0;
}