[R] An infinite recursion error please explain!

[mousy0815]

Probability <- function(N, f, w, b, y, t, q) {
	#N is the number of lymph nodes
	#f is the fraction of Dendritic cells (in the correct node) that have the
antigen
	#w is time in terms of hours
	#b is the starting position (somewhere in the node or somewhere in the gap
between nodes. It is a number between 1 and (x+t))
	#y is the number of time steps it takes to traverse the gap (8hr/y)
	#t is the number of time steps it takes to traverse a node. (24hours total
-- so 24hr/t)
	#q is the length of the time step
	
		m <- ceiling(w/q)
		A <- 1/N
		B <- 1-A
		C <- 1-f
		D <- (((m+b-1)%%(y+t))+1)
		
	

		if (b<=t) {########starts inside node
		if (m<=(t-b)){return(B + A*(C^m))} # start & end in first node		
		if (D<=t) { # we finish in a node
			a <- (B + A*(C^(t-b))) #first node
			b <- ((B + A*(C^t))^(floor((m+b)/(y+t))-1))  # intermediate nodes (if
any)
			c <- (B + A*(C^D))  # last node  
			return(a*b*c)
			} else {return(Probability(N, f, ((m*q)-q), b, y, t, q))} ## finish in a
gap	
		} else {###### starts outside node
			if (m<=(y+t-b)) {return(1)} #also end in the gap
			if (D<=t) { #end in a node
				b <- ((B + A*(C^t))^(floor((m/(y+t)))))
				c <- (B + (A*(C^D)))
				return(b*c)
				} else {return(Probability(N, f, ((m*q)-q), b, y, t, q))} #outside node
		}
	}	


This works for most values of 'w' (time):

> Probability(100, 0.001, 1, 1, 20, 20, (10/60))
[1] 0.9999401
> Probability(100, 0.001, 2, 1, 20, 20, (10/60))
[1] 0.9998807
> Probability(100, 0.001, 3, 1, 20, 20, (10/60))
[1] 0.9998215
> Probability(100, 0.001, 4, 1, 20, 20, (10/60))
Error: evaluation nested too deeply: infinite recursion /
options(expressions=)?

But once I get to w=4 I get an infinite recursion. I get that there is a
recursion in my function but I'm not sure why it wouldn't work. After a
certain point (D<=t) would be true. 
Any help, please?

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