# -*- coding: utf-8 -*-
# https://yukicoder.me/problems/134

def main():
    l, k = map(int, input().split())
    n = 2 * k   # 一回のかじりで減る量
    m = l // n if l % n != 0 else l // n - 1
    print(m * k)

if __name__ == '__main__':
    main()