function Main(input) {
    let src = input.replace("\n", "").split(" ");
    let n = parseInt(src[0]);
    let k = parseInt(src[1]);
    let ans = 0;
    if (!(k == 0 || k > n)) {
        if (n % 2) {
            if (k == (n + 1) / 2) {
                ans = n - 1;
            } else {
                ans = n - 2;
            }
        } else {
            ans = n - 2;
        }
    }
    console.log(ans);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));