Linked List Help

[pnoylover]

can anyone help me get an idea on how to add or subtract numbers in linked list
for example i input the number 1,3,5 in order
the display will 135
then i input the plus sign(+)
then the input will 45
it will always add the 1st and 2nd number from the left

here's my work so far:

#include<stdio.h>
#include<conio.h>
#include<malloc.h>
#include<stdlib.h>
#include <string>

using namespace std;

struct node{
       string o;
       int data;
       struct node *link;
       };
       typedef struct node *nodepointer;
       
       nodepointer add(nodepointer &head, int num);
       void del(nodepointer &head);
       nodepointer ex(nodepointer head);
       void display(nodepointer head);
      
       int main()
       {
           nodepointer head=NULL;
           int a,num,sum=0;
           string o;
           
           do{
               system("CLS");
               printf("[1]Add a number in front \n");
               printf("[2]Mathematical Operation \n");
               printf("[3]Display \n");
               printf("[0]Exit\n");
               printf("Enter your choice: ");
               scanf("%i",&a);
               
               switch(a)
               {
                        case 1: 
                             printf("Enter a number: ");
                             scanf("%i",&num);
                             head = add(head,num);
                             getch();
                             break;
                        case 2:
                             printf("Enter an operator: ");// +,-,*,/ 
                             scanf("%s",&o);
                             head = add(head,num);
                             getch();
                             break;
                        case 3:
                             display(head);
                             getch();
                             break;
                        case 0:
                             break;
                             }
                             }while(a!=0);
               getch();
               }

nodepointer add(nodepointer &head, int num)
    {
         nodepointer newnode;
         newnode=(nodepointer)malloc(sizeof(struct node));
         newnode->data=num;
         newnode->link=head;
         head=newnode;
         return head;
         }
/*
nodepointer ex(nodepointer head)
{
            if(head == NULL)
            {
                     printf("Invalid");
            }
            else
            {
                while(head->link!=NULL)
                {
                                       head+= newnode;
                }
            }
}
*/                                                    
void display(nodepointer head)
       {
            if(head==NULL)
            {
                          printf("\n no data");
                          }
                          else
                          {
                          while(head->link!=NULL)
                          {
                                                 printf("%i",head->data);
                                                 head=head->link;
                                                 }
                                                 printf("%i",head->data);
                                                 }    
                                                                   
}