minimum in doubly

#include<stdio.h>
#include<stdlib.h>
typedef struct node
{
    int a;
    struct node *next, *prev;
}node;

node *head=NULL, *tail=NULL;

void insert_at_last()
{
    node *N=(node*)malloc(sizeof(node));
    scanf("%d", &N->a);
    N->next=NULL;

    if(head==NULL)
    {
        head=N;
        tail=N;
        return;
    }
    else
    {
        tail->next=N;
        N->prev=tail;
    }
}

void min()
{
    int min=0;
    node *list=head;
    if(head==NULL)
    {
        printf("This is Empty.\n");
        return;
    }
    else
    {
        while(list!=NULL)
        {
            if(list->a<min)
            {
                min=list->a;
            }
            list=list->next;
        }
    }
    printf("Min: %d\n", min);
}

int main()
{
    int i,x;
    for(i=1; i<=4; i++)
    {
        insert_at_last();
    }
    min();
}