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