Sum of all the multiples of 3 or 5 below 1000 gives a wrong answer in C

Question

Project Euler problem:

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9

Answer 1

Direct translation of your python code:

#include 

int main(int argc, char *argv[])
{
  int sum = 0;

  for (int x = 0; x < 1000; x++)
  {
    if (x % 5 == 0 || x % 3 == 0)
      sum += x;
  }

  printf("%d", sum);
}