import java.math.BigInteger;
import java.util.Scanner;

public class bigKnum {

    public static void main(String args[]){
        Scanner in=new Scanner(System.in);
        int n=in.nextInt();
        int k=in.nextInt();

        BigInteger f1,f2,f3;
        f1=BigInteger.valueOf(k-1);
        f2=f1.multiply(BigInteger.valueOf(k));

        for(int i=2;i<n;i++)
        {
            f3=(f1.add(f2)).multiply(BigInteger.valueOf(k-1));
            f1=f2;
            f2=f3;

        }
        System.out.print(f2);

    }

}

Instead of using three temporary variables f1, f2, f3 make one array of three BigInteger elements f and just index it using modulo like so f[i%3], f[(i-1)%3], f[(i-2)%3]. This works exactly the same, and you don't have to make this awful and confusing data move of f1=f2; f2=f3;