function Main(input) {
    input = input.split(" ");
    var N = input[0];
    var D = input[1];
    var c2 = 0; 
    while (N > 0){
        N = N - (D * 2 + 1)
        c2++
    }
    console.log(c2)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));