This is a program for getting the sum of prime numbers of given range. The problem is that it would take hours if the limit is about 1000000. I've read about the sieve of Eratosten but having difficulties with implementing it. Could someone help to make this piece of code working faster. Best regards.

```
#include <stdio.h>
#include <math.h>
void prime_num( int num)
{
double sum = 0;
bool isPrime=true;
for ( int i = 2; i <= num; i++)
{
for ( int j = 2; j <= num; j++)
{
if ( i!=j && i % j == 0 )
{
isPrime=false;
break;
}
}
if (isPrime)
{
printf("\nPrime: %d",i);
sum = sum + i;
}
isPrime=true;
}
printf("\nThe sum of primes is: %.0lf\n",sum);
}
int main(){
printf("Enter a number and I will generate the prime numbers up to that number: ");
int num = 0;
scanf("%d",&num);
prime_num(num);
return 0;
}
```