Extract writing from colorful background

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Hey, im trying to extract a handwritten sum from a colorful background. So as a result i'd like to have a binary image with the sum black and the colorful background should be all filtered out and set to white.

The image looks like this:

I tried various treshholds, grabcut, color filters and morphologic operations to improve quality but i cant get a decent result.

I could maybe filter for "only" black in HSV color space, but in other images the sum might be written in blue. Maybe someone knows how to get the best possible result here.

Best regards