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


def pokki(L, K):
    yu = 0

    while(L >= 0):
        if L > K * 2:
            L = L - K * 2
            yu = yu + K
        else:
            break

    return yu


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

    print(pokki(L, K))


if __name__ == '__main__':
    main()