# 99 Elm Problems/Problem 37

Calculate Euler's totient function phi(m) (improved).See problem 34 for the definition of Euler's totient function. If the list of the prime factors of a number m is known in the form of problem 36 then the function phi(m) can be efficiently calculated as follows: Let ((p1 m1) (p2 m2) (p3 m3) ...) be the list of prime factors (and their multiplicities) of a given number m. Then phi(m) can be calculated with the following formula:

**# # # THIS IS A STUB # # #**

Example in Elm:

```
import Html exposing (text)
import List
f : Int -> Int
-- your implementation goes here
main = text (toString (f 0))
```

Result:

```
4
```