import sys


def resolve(in_):
    L, K = map(int, next(in_).split())
    
    K2 = K * 2
    d, m = divmod(L, K2)
    if not m:
        d -= 1
    ans = d * K

    return ans


def main():
    ans = resolve(sys.stdin)
    print(ans)


if __name__ == '__main__':
    main()