M for Mystery

M(4) = 1 + M(2)
M(2) = 1 + M(1)
M(1) = 1 + M(0)
M(0) = 2

plug in M(4) = 1 + 1 + 1 + 2 = 5

M(18) = 1 + M(9)
M(9) = 1 + M(4) 

M(18) = 1 + 1 + 5 = 7
M(18) = 1 + ( 1 + 5))