import std.stdio;
import std.conv;
import std.array;

void main()
{
    auto reader = readln.split;
    int N = reader[0].to!int;
    int D = reader[1].to!int;

    int range = D * 2 + 1;
    int ans = N / range;
    if (N % range > 0){
        ans++;
    }

    writeln(ans);
}

