"use strict";
function Main(input) {
    const PockeyLength = Number(input.split(" ")[0]);
    const BiteLength = Number(input.split(" ")[1]);
    let AteLength = 0;
    do {
        AteLength += BiteLength;
    } while (PockeyLength > AteLength * 2);
    console.log(AteLength - BiteLength);
}
//テストの入力を受け取る
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
//Main("100 50");
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