Minus-One
It took a long time for the notion of minus to make its appearance on the scene.

There were 5 people in the room, 7 leave, how many have to enter before the room is empty.

That is why it was a difficult notion. Also the Greeks were geometers and as such measures were  lengths  so there was no need for  negative numbers.

As recently as the 18th century,  Leonhard Euler believed that negative numbers were greater than infinity, and it was common practice to ignore any negative results returned by equations on the assumption that they were meaningless.
 


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