package codeup100;

import java.util.Scanner;

public class c1089 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

String[] nums = sc.nextLine().split(" ");

sc.close();

int a = Integer.parseInt(nums[0]);

int d = Integer.parseInt(nums[1]);

int n = Integer.parseInt(nums[2]);

int sum=0;

for(int i=0; i<n ; i++){

sum = a+(d*i);

}

System.out.println(sum);

}

}