YES(?, 20)

Initial complexity problem:
1:	T:
		(1, 1)    f0(a, b) -> f5(2, 4)
		(?, 1)    f5(a, b) -> f5(a + 2, a + 4) [ 19 >= a ]
		(?, 1)    f5(a, b) -> f8(a, b) [ a >= 20 ]
	start location:	f0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(1, 1)    f0(a, b) -> f5(2, 4)
		(?, 1)    f5(a, b) -> f5(a + 2, a + 4) [ 19 >= a ]
	start location:	f0
	leaf cost:	1

A polynomial rank function with
	Pol(f0) = 18
	Pol(f5) = -V_1 + 20
orients all transitions weakly and the transition
	f5(a, b) -> f5(a + 2, a + 4) [ 19 >= a ]
strictly and produces the following problem:
3:	T:
		(1, 1)     f0(a, b) -> f5(2, 4)
		(18, 1)    f5(a, b) -> f5(a + 2, a + 4) [ 19 >= a ]
	start location:	f0
	leaf cost:	1

Complexity upper bound 20

Time: 0.122 sec (SMT: 0.118 sec)