We know that the area of the half-circle divided by the area of the whole square multiplied by 100 gives us the percentage that the half-circles occupies in the square. Then, 100% minus the percentage found gives us the percentage of the rest of the square, which is the probability we're looking for. But I've tried to find a concrete percentage but I couldn't, I'm a bit rusty tonight and it's getting late Tell me if you get anything new...
Those are the calculations for what I've stated above:
% = The percentage that the half-square occupies
100% - %= The probability we're looking for...
100% - 129% = -29%