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I'm a first year student in a programming university and my first assignment is to find the sum of prime numbers between 3990000000 and 4010000000. The problem is everything I do, when I run the program it says the sum is 0 with a return value of 25. I've been trying to debug this code but with no luck, could someone help me?

My code is:

#include <stdio.h>
#define STARTNUMBER 3990000000 
#define ENDNUMBER 4010000000

int main() 
 unsigned int num;
 int j, c, flag, sum = 0;
 flag = 1;
 c = 5;
 j = 7;
 for (num = STARTNUMBER; num <= ENDNUMBER; num++) 
 if (num % 2 == 0) /*if number mod 2 equals zero go to next number*/
 flag = 0;
 break;
 
 if (num % 3 == 0) /*if number mod 3 equals zero go to next number*/
 flag = 0;
 break;
 else
 /*check if number is prime with the sequences 5+6+6...<=sqrt(number) and 7+6+6..<=sqrt(number)*/
 while (c * c <= num && j * j <= num && flag == 1) num % j == 0) 
 flag = 0;
 break;
 
 c += 6;
 j += 6;
 
 if (flag == 1)
 sum++;
 
 printf("There are %d prime numbers", sum);

You are asking for the sum of the prime number, even if your code is just printing how many they are. Assuming you've misunderstood the exercise, I try to show a possible problem of your original question, glimpsing a possible trick in your exercise since the interval is very close to 2³².

Assuming also you are on a 64-bit environment, if there are at least two prime numbers in that inteval, the sum is going to be greater than INT_MAX (2³¹ - 1). An int is not sufficient to store a value, and also unsigned int is not sufficient since UINT_MAX is 2³² - 1.

Finally, assuming you've solved your problems with the break statements already described in the comments, try to store your sum variable into a unsigned long int, and replace the last part of the loop with

if (flag==1)
 sum += num;