#!usr/bin/env python
# -*- coding: utf-8 -*-

import math


def pokki(L, K):
    yu = ((L - 1) / (K * 2))

    return math.floor(yu) * K


def main():
    L, K = map(int, input().split())

    print(pokki(L, K))


if __name__ == '__main__':
    main()