title: Get first element of Series without knowing the index
body: <p>Is there any way to access the first element of a Series without knowing its index?</p>
  <p>Let's say I have the following Series:</p>
  <pre><code>import pandas as pd
  
  key='MCS096'
  SUBJECTS = pd.DataFrame(
      {
          &quot;ID&quot;: pd.Series([146], index=[145]),
          &quot;study&quot;: pd.Series([&quot;MCS&quot;], index=[145]),
          &quot;center&quot;: pd.Series([&quot;Mag&quot;], index=[145]),
          &quot;initials&quot;: pd.Series([&quot;MCS096&quot;], index=[145]),
      }
  )
  </code></pre>
  <p>Print out <code>SUBJECTS</code>:</p>
  <pre><code>print(SUBJECTS[SUBJECTS.initials==key]['ID'])
  &gt;&gt;&gt; 145    146
  &gt;&gt;&gt; Name: ID, dtype: int64
  </code></pre>
  <p>How can I get the value 146 here without using index 145?</p>