Pointers as function returns and use of malloc

I am using Pointers as function returns ..below is a simple piece of code

main function:

void main()
{
    int a = 10, b = 20;
    int *ptr;
    ptr = add(&a, &b);
    printf("sum of a and b is %d\n", *ptr);
}

add function:

int* add(int *a, int *b)
{
    int c;
    c = *(a)+*(b);
    return &c;
}

This works correctly and gives me output 30..

But if you add one more function printhelloworld(); before add like below

void main()
{
    int a = 10, b = 20;
    int *ptr;
    ptr = add(&a, &b);

    printhelloworld();--this just prints hello world

    printf("sum of a and b is %d\n", *ptr);
}

output will not be 30 any more and it is undefined due to stack frame getting freed..so I have to modify my program like below using malloc()

int* add(int *a, int *b)
{    
    int* c = (int*)malloc(sizeof(int));
    *c = *(a)+*(b);
    return c;
}

This works.

But if I don't free memory allocated in heap, won't it stay forever ? Should I not use free() like below ?

free(c);

If I use free() in main,c is not in scope and it won't work, if I use 'free` in add, again I will get undefined result.

ASK:
What is the correct way to use free() in my case

free(C);

Total program for repro

#include<stdio.h>
#include<stdlib.h>

void printhelloworld()
{

    printf("hello world\n");
}

int* add(int *a, int *b)
{

    int* c = (int*)malloc(sizeof(int));

    *c = *(a)+*(b);
    //free(c);
    return c;
}


int main()
{

    int a = 10, b = 20;

    int *ptr;
    ptr =(int*) malloc(sizeof(int));
    ptr = add(&a, &b);

printhelloworld();
printf("sum of a and b is %d\n", *ptr);

}