"use strict";
const Main = (input) =>{
   let xs = input.trim().split(' ').map(v=>parseInt(v,10));
   let ans = xs[0] + xs[1];
   if(ans%2 == 0){
     ans = ans/2;
     console.log(ans);
   }else{
     console.log('IMPOSSIBLE');
   }   
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));