function Main(input) {
	input = input.split("\n");
	input1 = input[0].split(" ")
	const A = parseInt(input1[0])
	const B = parseInt(input1[1])
	if((A+B) % 2 == 1){
		console.log("IMPOSSIBLE")
	}else{
		console.log((A+B)/2)
	}
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));