title: How to add new column with the length of the values and the custom start?
body: <p>There are 2 dataframes</p>
  
  <p>df1 and df2</p>
  
  <pre><code>df1 =
  
  A   B
  111 222
  222 5555
  
  df2 =
  
  C   counter_2
  333 100
  777 25
  </code></pre>
  
  <p>I need to add a column "counter" to df1, it should just be like index, return the position of column's elements.</p>
  
  <p>Like this:</p>
  
  <pre><code>df1['counter'] = range(len(df1))
  
  A   B   counter
  111 222       1
  222 555       2
  </code></pre>
  
  <p>But I need to change the starting point, it should be the max number from df2 "counter" column. </p>
  
  <pre><code>df2['counter_2'].max() # 100
  </code></pre>
  
  <p>So my desired output is like:</p>
  
  <pre><code>A   B   counter
  111 222       101
  222 555       102
  </code></pre>
  
  <p>I've googled, but I can't find a solution. </p>