y=2*x
x=n*L
L=a+a' 
(a: the distance from the meeting point back to the starting point of the loop, 
a': the distance from the meeting point proceed to the starting point of the loop)
b: the distance between the starting point of the loop and the beginning
x=b+a
b=n*L-a
b=n*L-(L-a')
b=(n-1)*L+a'