Sum of numbers

Write a program that reads in positive integers from
the user, each separated by a space, and the whole
terminated by a negative number (e.g. 3 22 10 -2),
and returns the sum of the numbers entered (not
including the terminated negative number!).
#include "si204.h"

int main() {
  // initialization
  fputs("Enter numbers separated by spaces to add up.\n", stdout);
  fputs("Use a negative number to indicate you're finished.\n", stdout);

  int sum = 0;
  int k;

  k = readnum(stdin);
  // Loop while the user enters more data
  while (k >= 0) {
    sum = sum + k;
    k = readnum(stdin);

  // write out result
  fputs("The sum is ", stdout);
  writenum(sum, stdout);
  fputs("\n", stdout);

  return 0;