import java.util.Scanner;

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

        String s = sc.nextLine();

        long N = sc.nextInt();

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

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

        for(int i =0; i<str.length; i++){
            num[i] = Long.parseLong(str[i]);
        }
        
        long diff = 0;
        long rst = 0;

        if(num.length>1){
            diff = num[1] - num[0];
        }
        

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

}
}