1 degree is equal to pi/180 radians.
1 radian is equal to 180/pi degrees.
From here we can see that pi radians is 180 degrees.
Remember that negative angles are angles that go clockwise.
pi radians is the same as -pi radians because going 180 degrees counterclockwise is the same as going 180 degrees clockwise. You can imagine this.
Therefore -pi radians is 180 degrees.
For angles in general, -r radians is the same angle as 2pi - r radians. Similarly, -r degrees is the same angle as 360-r degrees. You can do this if you want to work with positive angles.