function main(input) {
    var args = input.trim().split(" ");
    var A = Number(args[0]);
    var B = Number(args[1]);

    var total = A + B;

    if (total % 2 !== 0) {
        console.log("IMPOSSIBLE");
        return;
    }

    console.log(total / 2);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));