Here is a definition that includes a comment explaining what it is doing. You are not required to include such a comment.

  int sum(int a, int b)
  {
    int s = 0;
    int k = a;

    //------------------------------------------------------------
    // Every time the program reaches the top of the following loop,
    //   s = the sum of all integers that are greater than or
    //       equal to a and strictly less than k
    // (So s = a + ... + (k-1).)
    //------------------------------------------------------------

    while(k <= b) 
    {
      s = s + k;
      k = k + 1;
    }
    return s;
  }