import java.util.Scanner;

public class sequenceNumber {
     public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String s = sc.nextLine();

        int N = sc.nextInt();

        s = s.replace(".", "");
        String [] str = s.split("\\+");

        int []num = new int[str.length];

        for(int i =0; i<str.length; i++){
            num[i] = Integer.parseInt(str[i]);
        }

        int diff = num[1] - num[0];
        int rst = 0;
        

        for(int i=0; i<N; i++){
            rst += num[0] + i*diff;
        }
     
        System.out.println(rst);

}
}