def brute_force(s: str, n: int) -> int:
    nums = parse_nums(s)
    a = nums[0]
    d = nums[1] - nums[0]
    total = 0
    cnt = 0
    val = a
    while cnt < n:
        total += val
        val += d
        cnt += 1
    return total

s = line
n = int(sys.stdin.readline().strip())
brute = brute_force(s, n)
print (brute)