def main():
    L, K = map(int, input().split())
    half_length_of_pokky = L / 2
    bite_count = half_length_of_pokky // K
    you_bite = K * bite_count
    if bite_count <= 0:
        print(0)
    elif you_bite == half_length_of_pokky:
        print(int(K * (bite_count - 1)))
    else:
        print(int(K * bite_count))
        

if __name__ == '__main__':
    main()