#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
    int a;
    int b;
    int i;
    int res = 0;
    cin >> a;
    cin >> b;

    for(i = 0; i<1000000000; i++){

            if(abs(a-i) == abs(b-i)){
                res = i;
                break;
            }
    }

                if(res>0){
                    cout<< i;
                }
                else
                    cout<< "IMPOSSIBLE";
}
