title: time complexity when while not execute
body: <p>What is the “best case” time complexity of the following segment of program?</p>
  
  <pre><code>n=0
  sum=0
  input(x)
  while x!=-999 do
    n=n+1
    sum=sum+x
    input(x)
  end {while}
  mean=sum/n
  </code></pre>
  
  <p>Does "best case" could be O(1) when user in the first time type “-999” 
  note: when user type -999 in first time, "mean" will be 0/0, result of function is undefined</p>