YES(?, b + 3)

Initial complexity problem:
1:	T:
		(?, 1)    f2(a, b, c, d, e, f) -> f2(a - 1, b - 1, a, b, a - 2, f) [ a >= 1 /\ b >= 1 ]
		(1, 1)    f3(a, b, c, d, e, f) -> f2(a, b, c, d, e, f)
		(?, 1)    f2(a, b, c, d, e, f) -> f4(a, g, c, d, e, h) [ 0 >= b /\ 0 >= g ]
		(?, 1)    f2(a, b, c, d, e, f) -> f4(a, b, c, d, e, h) [ b >= 1 /\ 0 >= a ]
	start location:	f3
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    f2(a, b, c, d, e, f) -> f2(a - 1, b - 1, a, b, a - 2, f) [ a >= 1 /\ b >= 1 ]
		(1, 1)    f3(a, b, c, d, e, f) -> f2(a, b, c, d, e, f)
	start location:	f3
	leaf cost:	2

A polynomial rank function with
	Pol(f2) = V_2
	Pol(f3) = V_2
orients all transitions weakly and the transition
	f2(a, b, c, d, e, f) -> f2(a - 1, b - 1, a, b, a - 2, f) [ a >= 1 /\ b >= 1 ]
strictly and produces the following problem:
3:	T:
		(b, 1)    f2(a, b, c, d, e, f) -> f2(a - 1, b - 1, a, b, a - 2, f) [ a >= 1 /\ b >= 1 ]
		(1, 1)    f3(a, b, c, d, e, f) -> f2(a, b, c, d, e, f)
	start location:	f3
	leaf cost:	2

Complexity upper bound b + 3

Time: 0.189 sec (SMT: 0.180 sec)