Two things decide every walk. The sign of the second number decides which way Mazarine faces. Plus or minus between the numbers decides whether she walks forward or backpedals (steps backward without turning around).
Example
What she does
Where she goes
5 + 3 = 8
faces the positive side, walks forward
right, up 3
5 + (−3) = 2
faces the negative side, walks forward
left, down 3
5 − 3 = 2
faces the positive side, backpedals
left, down 3
5 − (−3) = 8
faces the negative side, backpedals
right, up 3
Look at the last row. She faces the negative side and steps backward, so she moves away from the negatives, toward the positives. She never turns around. That is why subtracting a negative number is the same as adding a positive one.