import java.util.Scanner;
class main{
    public static void main(String [] args){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int k=sc.nextInt();
        if (n<=0||k<=0){
            System.out.println(-1);
        
            return 0;
            
        }
        int totalCups =n;
        int freeCups=n/k;
        while (freeCups>0){
            totalCups+=freeCups;
            freeCups = freeCups/k;
        }
        System.out.println(totalCups);
        
    }
}