ℜ0, -180 is the same rotation as

ℜ 0, 180

ℜ -90, 180

ℜ 90, 180

ℜ 0, 90

Since 0, -180 is the rotation and if you look at the other rotations like 90, 180 the first rotation would be different. so 0, 180 is the same as 0, -180 because they both would be a full rotation of a 180 rotation. The first number "0 ," would have to be the same.