#!/usr/bin/env python3
from collections import defaultdict,deque
from heapq import heappush, heappop
import sys, math, bisect, random, itertools
sys.setrecursionlimit(10**5)
input = sys.stdin.readline
sqrt = math.sqrt
bisect_left = bisect.bisect_left
bisect_right = bisect.bisect_right
def LI(): return list(map(int, input().split()))
def LF(): return list(map(float, input().split()))
def LI_(): return list(map(lambda x: int(x)-1, input().split()))
def II(): return int(input())
def IF(): return float(input())
def LS(): return list(map(list, input().split()))
def S(): return list(input().rstrip())
def IR(n): return [II() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def FR(n): return [IF() for _ in range(n)]
def LFR(n): return [LI() for _ in range(n)]
def LIR_(n): return [LI_() for _ in range(n)]
def SR(n): return [S() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
mod = 1000000007
inf = float('INF')

#A
def A():
    n = II()
    print(n*n*3)
    return

#B
def B():
    n, d = LI()
    print((n-1)//(2*d+1)+1)
    return

#C
def C():
    n = II()
    a = IR(n)
    b = a[::1]
    b.sort()
    for i in range(n):
        if a[i] == b[-1]:
            print(b[-2])
        else:
            print(b[-1])
    return

#D
def D():
    return

#E
def E():
    return

#F
def F():
    return

#Solve
if __name__ == '__main__':
    B()
