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