Traceback (most recent call last): File "src.py", line 11, in <module> main() File "src.py", line 9, in main a=dist(x1,y1,x2,y2) File "src.py", line 2, in dist l=((x2_x1)**2 (y2-y1)**2)**0.5 NameError: name 'x2_x1' is not defined