The StackOverflowException occurs because it is an infinite recursion. Take a pen and paper and try to solve for F(1), you'll find that this happens:
Code:
F(1) = F(1 - 2) + 2 which simplifies to F(-1) + 2
F(-1) = F(-1 - 2) + 2 which simplifies to F(-3) + 2
F(-3) = F(-3 - 1) - 1 which simplifies to F(-4) - 1
F(-4) = F(-4 - 1) - 1 which simplifies to F(-5) - 1 
F(-5) = F(-5 - 2) + 2 which simplifies to F(-7) + 2
'To infinity and beyond