Delete and insert from linked list

#include <iostream>

using namespace std;

struct Node
{
    int data;
    struct Node* next;
} *first = NULL, *last = NULL;

void add(int x)
{
    if(first == NULL)
    {
        first = new Node;
        first->data = x;
        first->next = NULL;
        last = first;
    }
    else
    {
        Node *t = new Node;
        t->data = x;
        t->next = NULL;
        last->next = t;
        last = t;
    }
}

void display (struct Node* p)
{
    while(p)
    {
        cout << p->data << " ";
        p = p->next;
    }
}

void deleteNode (int key)
{
    Node* prevNode = NULL;
    Node* p = first;
    if(key == first->data)
    {
        Node *t = first;
        first = first->next;
        delete t;
    }
    else
    {
      while(p->data!=key)
        {
            prevNode = p;
            p = p->next;
        }
    }
    Node *t = p;
    prevNode->next = p->next;
    delete p;
}

void insertAfterNode(int key, int value)
{
    Node *p = first;
    Node *previousNode = NULL;
    if(first->data == key)
    {
        Node *t = new Node;
        t->data = value;
        t->next = first->next;
        first->next = t;
    }
    else
    {
        while(p->data!=key)
        {
            p = p->next;
        }
        Node *t = new Node;
        t->data = value;
        t->next = p->next;
        p->next = t;


      }
}

int main()
{

    add(24);
    add(75);
    add(3);
    add(10);
    display(first);
    cout << endl;
    //deleteNode(3);
    insertAfterNode(24,2);
    display(first);
}