m = list(map(int,input().split()))
L = m[0]
K = m[1]
i = 0
M = L
while L > 0:
    L -= 2 * K
    if L <= 0:
        print(i * K)
        break 
    elif L - 2 * K > 0:
        i = i + 1
    elif L == 0:
        print(i * K)
    elif L - 2 * K <= 0:
        i = i + 1
        print(i * K)
        break