
function Main(input) {
    var a = parseInt(input[0], 10);
    var b = parseInt(input[1], 10);

    var result = (a + b) / 2;
    console.log((a + b) % 2 === 0 ? result : 'IMPOSSIBLE');
}

Main(require("fs").readFileSync("/dev/stdin", "utf8").trim().split(/\n|\s/));