import io
import sys

sys.stdin = io.StringIO('''2 3 1 3''')  # 26

a, h, b, k = map(int, input().split())
c = 0
while h > 0 and k > 0:
    c += a
    c += b
    h -= b
    k -= a

if h > 0 > k:
    c += a * 10
elif h <= 0 < k:
    c += b * 10

print(c)