input: Write a function to calculate the sum of all values in a DataFrame.
  assert sum_dataframe(pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 3]})) == 18
  assert sum_dataframe(pd.DataFrame({'X': [10, 20, 30], 'Y': [5, 15, 25]})) == 105
  assert sum_dataframe(pd.DataFrame({'M': [7, 8], 'N': [3, 2]})) == 20
output: def sum_dataframe(df):
      return df.to_numpy().sum()