import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD, inf
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines

L,K=map(int,readline().split())
a,b=divmod(L,K)
if a%2==0 and b==0:
    a=a//2-1
else:
    a//=2
ans=a*K
print(ans)