title: Summing over filtered pivot table with pandas / python
body: <p>I have a (sliced [by years 1952-1988] and countries [USA and USR]) pivot table of olympic medals with countries (noc), years and corresponding amount of medals:</p>
  
  <pre><code>medals_of_usa_urs = df.loc['1952':'1988', ['USA','URS']]
  print(medals_of_usa_urs )
  
  noc     USA    URS
  year              
  1952  130.0  117.0
  1956  118.0  169.0
  1960  112.0  169.0
  1964  150.0  174.0
  1968  149.0  188.0
  1972  155.0  211.0
  1976  155.0  285.0
  1980    NaN  442.0
  1984  333.0    NaN
  1988  193.0  294.0v
  </code></pre>
  
  <p>How can I sum up total medals by country (e.g. for USA a toal of 1495 and URS 2049) in Python?</p>