function Main(input) {
    input = input.split("\n")[0];
    tmp = input.split(" ");
    an = Number(tmp[0])/2 + Number(tmp[1])/2;
    if(!Number.isInteger(an)) console.log("IMPOSSIBLE");
    else console.log(an);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
