Finding P(S1 = 1 | D1=1, D2=1) (Bayes Networks)

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I'm scratching my head over something that is probably simple Probabilities, but I guess I can't see it. Essentially, I have the following table:

 ------ ------ ------ ------
|  D1  |  D2  | S1=1 | S1=0 |
 ------ ------ ------ ------
|  0   |  0   |  0   |  1   |
 ------ ------ ------ ------
|  0   |  1   |  0.8 |  0.2 |
 ------ ------ ------ ------
|  1   |  0   | 0.75 | 0.25 |
 ------ ------ ------ ------
|  1   |  1   |  ?   |  ?   |
 ------ ------ ------ ------

Where, for example, the 0.8 in line 3 column 3 is the value for P(S1 = 1 | D2=1). I'm having trouble finding The ones marked with ?, namely P(S1=1 | D1=1, D2=1) (the other one is trivially 1-P(S1=1 | D1=1, D2=1)). Any help?

Thank you for your time in advance. I'll provide more details if necessary!