Why?
This is my solution:

#include <iostream>
using namespace std;
int main(){
    long int a,b,f=0;
    long int k=1;
    cin>>a>>b;
    if (a==1) {
           f=0;
           cout<<f;
           return 0;

           }

    if (b>=a && a<=2)
            f=1;

    else if(a!=1){
    while (k<=b){
          f++;
          k*=2;

          }
    a -=k;
    while (a>0){
          f++;
          a=a-b;

          }
          }
          cout<<f;

    return 0;
    }

Some test for your program
10 1 ans 9
3 8  ans 2

Спасибо, помогло=)