"use strict";

const main = arg => {

    const input = arg.trim().split(" ");

    const a = parseInt(input[0]);
    const b = parseInt(input[1]);

    const mx = Math.max(a, b);
    const mn = Math.min(a, b);

    if ((mx - mn) % 2 == 0) {
        console.log((mx + mn) / 2);
    } else {
        console.log("IMPOSSIBLE");
    }
}

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