#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update
using namespace std;

#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize ("O3")
#pragma GCC optimize ("O2")
#pragma GCC optimize("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#define ll long long
#define pb push_back
#define mp make_pair
#define endl "\n"
#define sz(x) (int)x.size()
#define aint(v) v.begin(), v.end()
#define x first
#define y second
#define in(x) cin >> x;
#define inde(x) ll (x); cin >> x;
#define rep(ai, j, k) for(ll ai=j; ai<k; ai++)
#define sep(ai, j, k) for(ll ai=j; ai>=k; ai--)
#define ios ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define inputarr(x,y) for(int ai=0; ai<y; ai++) {cin >> x[ai];}
#define inpprefixarr(arr,y) cin>> arr[0]; for(ll ai=1;ai<y;ai++) {ll temp; cin >> temp; arr[ai]= temp + arr[ai-1];}

#define watch(x) cout << (#x) << " is " << (x) << endl
#define watcharrsz(x) cout << (#x) << " starts:\n"; for(int ai=0;ai<sz(x);ai++) {cout << x[ai]<<" ";} cout << "\n" << (#x) << "ends\n";
#define watcharr(x,n) cout << (#x) << " starts:\n"; for(int ai=1;ai<=n;ai++) {cout << x[ai]<<" ";} cout << "\n" << (#x) << "ends\n";
#define watchg(x,n,y) cout << (#x) << " starts:\n"; for(int ai=0; ai<=n; ai++) {cout << (ai) << ":"; for(int aj=0; aj<x[ai].size();aj++) {cout << x[ai][aj]<< y;} cout << "\n";}cout << "\n"<< (#x) << "ends\n";
#define watch2darr(x,m,n,y) cout << (#x) << " starts:\n"; for(int ai=1;ai<=m;ai++) { for(int aj=1;aj<=n;aj++) {cout << x[ai][aj]<< y;} cout << "\n";}cout << "\n"<< (#x) << "ends\n";

#define print(x,y) cout << (x) << y;
#define printarrsz(x,y) for(int ai=0; ai<sz(x); ai++) {cout << x[ai]<< y;} cout << "\n";
#define printarr(x,n,y) for(int ai=0; ai<n; ai++) {cout << x[ai]<< y;} cout << "\n";
#define print2darr(x,m,n,y) for(int ai=0;ai<m;ai++) { for(int aj=0;aj<n;aj++) {cout << x[ai][aj]<< y;} cout << "\n";}cout << "\n";
#define mod(x) = (x % m + m) % m;

const ll m = (ll)1000000007;
const ll N = 3e6+5;
const ll INF = 1ll<<63; 

typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef set<int> si;
typedef map<int,int> mii;
typedef pair<int,int> pii;
typedef vector<bool> vb;
typedef vector<pii> vpii;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef set<ll> sl;
typedef map<ll,ll> mll;
typedef pair<ll,ll> pll;
typedef vector<pll> vpll;

typedef priority_queue<pll, vpll, greater <pll> > pqg;
/***************************Code Begins Here *****************************/


void solve()
{
    // Never use x,y here, try to use i,j in loops only
    ll a,b;
    in(a)in(b);
    ll mid = abs(b-a);
    if(mid%2==0) {
        mid = mid/2;
        print(mid + min(a,b),endl);
    }
    else {
        print("IMPOSSIBLE",endl);
    }
}

signed main()
{
    ios
    ll t=1;
    // cin>>t;

// #ifndef ONLINE_JUDGE
//     time_t time_t1, time_t2;
//     time_t1 = clock();
// #endif

    while(t--)
        solve();

// #ifndef ONLINE_JUDGE
//     time_t2 = clock();
//     cout << "time taken :" << time_t2 - time_t1 << endl;
// #endif
    return 0;
}

Measured memory_bytes: 256