l,k = map(int,input().split())
if l%k == 0 and (l/k)%2 == 0:
    print((l/k/2-1)*k)
elif l%k == 0 and (l/k)%2 == 1:
    print((l/k//2)*k)
elif l%k != 0 and (l//k)%2 == 0:
    print((l//k/2)*k)
else:
    print((l//k//2)*k)