0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
Rp 7
Rp 28.000
Rp 187.500
Rp 1.234.567
fib( 5) =      5 after       15 calls
fib(10) =     55 after      177 calls
fib(20) =   6765 after    21891 calls
fib(25) =  75025 after   242785 calls
fib(30) = 832040 after  2692537 calls
