function main(arg) {
    const A = parseInt( arg.split(/\s/)[0] );
    const B = parseInt( arg.split(/\s/)[1] );
    var ans = (A+B)/2;
    if(ans==parseInt(ans)){
        console.log(ans);
    }else{
        console.log("IMPOSSIBLE");
    }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));