"use strict";
function Main(input) {
    const PockeyLength = Number(input.split(" ")[0]);
    const BiteLength = Number(input.split(" ")[1]);
    let ans;
    if (PockeyLength <= BiteLength * 2) {
        ans = 0;
    }
    else {
        const leftOver = PockeyLength - Math.floor(PockeyLength / BiteLength) * BiteLength;
        if ((PockeyLength - leftOver) / BiteLength % 2 === 1) {
            ans = (PockeyLength - leftOver - BiteLength) / 2;
        }
        else {
            if (leftOver === 0) {
                ans = (PockeyLength - leftOver) / 2 - BiteLength;
            }
            else {
                ans = (PockeyLength - leftOver) / 2;
            }
        }
    }
    console.log(ans);
}
//テストの入力を受け取る
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
//Main("101 5");
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