@Lician
For example finding fib(5):
Work through it from the top down as per the tree diagram below. When the function returns a value at the bottom, work your way back up replacing each function call with the value it returns and you will reach the answer which is 5.

                                  fib(5-1)                     +                      fib(5-2)
                  fib(4-1)           +          fib(4-2)                 fib(3-1)        +        fib(3-2)
      fib(3-1)       +     fib(3-2)       fib(2-1) + fib(2-2)       fib(2-1) + fib(2-2)               1
fib(2-1) + fib(2-2)            1              1     +    0              1     +     0               
    1    +    0  

I don't understand how it should work...

on fingers

how did you calculate it?)

thx) i'll remember this.

if the number is 100, then in this way the calculation will be 50,000 years