Where will the ball land?
Python, 143B
import sys
for l in sys.stdin:
 a=l.find('.')
 if a&gt;-1:F=a
 elif F&gt;-1: 
    if'\\/'in l[F-1:F+2]:z
    F+={'\\':1,'/':-1}.get((l+' '*F)[F],0)
print F+1

Using the space/tab indentation trick. I haven't done anything particularly clever here. F is the current index, l is the current line; z is undefined so it throws an exception, which is definitely not a positive integer, handling the \/ situation.