var $jscomp=$jscomp||{};$jscomp.scope={};$jscomp.arrayIteratorImpl=function(a){var b=0;return function(){return b<a.length?{done:!1,value:a[b++]}:{done:!0}}};$jscomp.arrayIterator=function(a){return{next:$jscomp.arrayIteratorImpl(a)}};$jscomp.makeIterator=function(a){var b="undefined"!=typeof Symbol&&Symbol.iterator&&a[Symbol.iterator];return b?b.call(a):$jscomp.arrayIterator(a)};
function main(a){var b=$jscomp.makeIterator(a.split(" "));a=b.next().value;b=b.next().value;a=parseInt(a);b=parseInt(b);return 0==(a+b)%2?console.log((a+b)/2):console.log("IMPOSSIBLE")}main(require("fs").readFileSync("/dev/stdin","utf8"));
