#include <iostream>
#include <bits/stdc++.h>
#include <algorithm>
#include <vector>
#include <string>
#include <cmath>
#include <set>
#include <utility>

using namespace std;

typedef long long ll;

int main()
{
    ios::sync_with_stdio(0);
    ll a,b;
   cin>>a>>b;
   if(a==0||b==0)
    cout<<"IMPOSSIBLE";
   else
   {
    if(a!=0&&b!=0)
  cout<<(b*b-a*a)/(2*(b-a));
   }
}
