code: def euler_totient(n):
      
      result = n;
      for i in range(2, int(n ** 0.5) + 1):
          if n % i == 0:
              while n % i == 0:
                  n //= i
              result -= result // i
      if n > 1:
          result -= result // n;
      return result;
docs: Euler's totient function or Phi function.
      Time Complexity: O(sqrt(n)).