"use strict";
function Main(input) {
    let PockeyLength;
    let BiteLength;
    [PockeyLength, BiteLength] = input.split(" ").map(i => Number(i));
    let ans;
    if (PockeyLength <= BiteLength * 2) {
        ans = 0;
    }
    else {
        const leftOver = PockeyLength - Math.floor(PockeyLength / BiteLength) * BiteLength;
        const harfPockeySize = (PockeyLength - leftOver) / 2;
        if (leftOver != 0) {
            ans = harfPockeySize;
        }
        else {
            ans = harfPockeySize - BiteLength / ((PockeyLength - leftOver) / BiteLength % 2 + 1);
        }
    }
    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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