func readInts() -> [Int]{
    return readLine()!.split(separator: " ").map{Int($0)!};
}
let a = readInts();
let N = a[0];
let D = a[1];
var range = -1, ans = 0;
while(range < N - 1) {
    range += (2 * D + 1);
    ans += 1;
}
print(ans);