#include <iostream>
using namespace std;
int main (){
    int a,b;
    cin>> a;
    cin>> b;
    if((a+b)%2!=0){
        cout<< "IMPOSSIBLE"<< endl;
    }
    else //if two numbers are equal it should be divisible by 2
        cout<<(a+b)/2<<endl;


return 0;
}
