L, K = map(int, input().split()) #Lはポッキー延長、Kは一回当たりの食べる長さ

if L % (K * 2) == 0:
    n = L / (K * 2) - 1
    l = K * n
    print(int(l))
elif L % (K * 2) != 0:
    n = L // (K * 2) 
    l = K * n
    print(int(l))