"use strict";

const input = require("fs").readFileSync("/dev/stdin", "utf8");
const inputs = input.split(" ");

const A = Number(inputs[0]);
const B = Number(inputs[1]);

const main = (A, B) => {
    let X;
    if ((A + B) % 2 === 0) {
        return Math.abs((A + B) / 2);
    } else {
        return "IMPOSSIBLE";
    }
};
console.log(main(A, B));