int sum(int n) {
  if (n < 0) return 0;
  return n + sum(n - 1);
}

int sum1(int n) {
  int i = 0, j = 0, sum = 0;
  for (; i <= n; i++,j++) sum += j;
  return sum;
}

int sum2(int n) {
  int i,sum=0;
  for (i=n;i>=0;i--) sum=sum+i;
  return sum;
}

int sum3(int n) {
  int ret = 0;
  for (int i = 0; i <= n; i++)
    for (int j = 0; j < i; j++)
      ret++;
  return ret;
}

