This is a recursive definition, meaning (in this case, at least) that in order to find the value of F(n), you must first find the values of F(n-1) and F(n-2), each of which requires another step of recursion, which requires another step and so on.

As you might imagine, this calculation is quite cumbersome. After a bit of working out, you might ask yourself: “is there a better way?” or perhaps more specifically, “is there a direct formula I can use to find the nth Fibonacci number?”