s = input().strip()
n = int(input().strip())

# Extract first two numbers from string
nums = []
tmp = ""
for c in s:
    if c.isdigit():
        tmp += c
    elif c == '+':
        if tmp:
            nums.append(int(tmp))
            tmp = ""
if tmp:
    nums.append(int(tmp))

# First term and difference
st = nums[0]
diff = nums[1] - nums[0]

# Brute force summation
cnt = 0
total = 0
i = st
while cnt < n:
    total += i
    i += diff
    cnt += 1

print(total)