#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;
    ll k=abs((b*b-a*a)/(2*(b-a)));
   cin>>a>>b;
   if(a%2==0&&b%2==1||a==b||a%2==1&&b%2==0)
    cout<<"IMPOSSIBLE";
   else
  cout<<k;
}
