import std.algorithm;
import std.array;
import std.container;
import std.conv;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

void scan(T...)(ref T a) {
    string[] ss = readln.split;
    foreach (i, t; T) a[i] = ss[i].to!t;
}
T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

int calc(int n, int d) {
    int ans = 1;
    int p = d + 1;
    while (p + d < n) {
        ans++;
        p += d * 2 + 1;
    }
    return ans;
}

void main() {
    int n, d; scan(n, d);
    writeln(calc(n, d));
}
