Polynomial Addition using Linked List

#include<stdio.h>
#include<stdlib.h>
struct node
{
   int coeff,expon;
   struct node *link;
};
typedef struct node *NODE;
NODE insert(NODE,int,int);
NODE add(NODE,NODE);
void display(NODE);
main()
{
   NODE poly1=NULL,poly2=NULL,poly3=NULL;
   int coef,expo,choice=0;
   printf("\n enter details of first polynomail\n");
   do
   {
       printf("\n enter coefficient:");
       scanf("%d",&coef);
       printf("\n enter exponent:");
       scanf("%d",&expo);
       poly1=insert(poly1,coef,expo);
       printf("\n enter 1 to continue:");
       scanf("%d",&choice);
   }while(choice==1);
   printf("\n enter details of second polynomail\n");
   do
   {
       printf("\n enter coefficient:");
       scanf("%d",&coef);
       printf("\n enter exponent:");
       scanf("%d",&expo);
       poly2=insert(poly2,coef,expo);
       printf("\n enter 1 to continue:");
       scanf("%d",&choice);
   }while(choice==1);
   printf("\nfirst polynomail entered is\n");
   display(poly1);
   printf("\nsecond polynomail entered is\n");
   display(poly2);
   poly3=add(poly1,poly2);
   printf("\n the sum of polynomails is\n");
   display(poly3);
}
NODE insert(NODE poly,int coef,int expo)
{
   NODE temp,cur;
   temp=(NODE)malloc(sizeof(NODE));
   temp->coeff=coef;
   temp->expon=expo;
   temp->link=NULL;
   if(poly==NULL)
   {
       poly=temp;
       return poly;
   }
   cur=poly;
   while(cur->link!=NULL)
    cur=cur->link;
   cur->link=temp;
   return poly;
}
NODE add(NODE poly1,NODE poly2)
{
   NODE poly3=NULL;
   while(poly1!=NULL&&poly2!=NULL)
   {
       if(poly1->expon>poly2->expon)
       {
           poly3=insert(poly3,poly1->coeff,poly1->expon);
           poly1=poly1->link;
       }
       else
       if(poly1->expon<poly2->expon)
       {
           poly3=insert(poly3,poly2->coeff,poly2->expon);
           poly2=poly2->link;
       }
       else
       {
           poly3=insert(poly3,poly1->coeff+poly2->coeff,poly1->expon);
           poly1=poly1->link;
           poly2=poly2->link;
       }
   }
   while(poly1!=NULL)
   {
       poly3=insert(poly3,poly1->coeff,poly1->expon);
       poly1=poly1->link;
   }
       while(poly2!=NULL)
   {
       poly3=insert(poly3,poly2->coeff,poly2->expon);
       poly2=poly2->link;
   }
   return poly3;
}
void display(NODE first)
{
   printf("%dX%d",first->coeff,first->expon);
   first=first->link;
   while(first!=NULL)
   {
       if(first->expon==0)
         {
           printf("+%d",first->coeff);
           break;
         }
         printf("+%dX%d",first->coeff,first->expon);
         first=first->link;
   }
}