Codewars Kata Attempt | Number of Proper Fractions with Denominator d | C++

kata.cpp

unsigned long long properFractions(unsigned long long n) {
  if(n == 1) return 0;
  unsigned long long d = n;
  for(unsigned long long i = 2; i * i <= d; i++) {
    if(d % i == 0) {
      n = n / i * (i - 1);
      while(d % i == 0) d /= i;
    }
  }
  if (d > 1) n = n / d * (d - 1);
  return n;
}