n,k = input().strip().split(' ')
n,k = int(n),int(k)
li = [int(x) for x in input().strip().split(' ')]
t=k*2+1
if(t>=len(li)):
    print(1)
    exit()
else:
    if( (n/t)>int(n/t) ):
        print(int(n/t)+1)
    else:
        print(int(n/t))