function main(input) {
    const args = input.split(' ');
    const a = parseInt(args[0], 10);
    const b = parseInt(args[1], 10);

    const k  = (a + b) / 2;
    if( Number.isInteger(k) ){
        console.log(k);
    }else {
        console.log('IMPOSSIBLE');
    }
}

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