Euler’s totient function

ll totient_function(ll n) 
{
    ll result = n;
    
    for (int i = 2; n/i >= i; ++i)
        if (n % i == 0) {
            while (n % i == 0)
                n /= i;
            result = result - result / i;
        }
    
    if (n > 1)
        result = result - result / n;
    
    return result;
}