const main = (A,B) => {
    K = Math.floor((A+B)/2)
    if(Math.abs(A-K) === Math.abs(B-K)) {
        return K
    }else{
        return "IMPOSSIBLE"
    }

}


const input = require('fs').readFileSync('/dev/stdin','utf8').split("\n")[0]
const AB = input.split(" ").map( x => Number(x))
console.log(main(AB[0],AB[1]))