function main(input) {
return (() => {
let a=null;
let b=null;
[a, b]=input.split(" ");
a=parseInt(a);
b=parseInt(b);
return ((((a+b)%2)==0) ? console.log(((a+b)/2)) : console.log("IMPOSSIBLE"));
})();
}

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