What is the running time of the following piece of code ?

int power(int b, int n)
{
     int p=1, q=b, m=n;
     while(m>0)
     {
          if(m%2!=0)
               p*=q;
          m/=2;
          q*=q;
     }
     return p;
}

Difficulty level
This exercise is mostly suitable for students 
O(log(n))
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