//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("unroll-loops")
 
#include <iostream>
#include <stdlib.h>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <assert.h>
#include <memory.h>
#include <time.h>
#include <bitset>
#include<iomanip>
 
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define ld long double
#define rep(i, l, r) for (int i = (l); i < (r); i++)
#define repb(i, r, l) for (int i = (r); i > (l); i--)
#define sz(a) (int)a.size()
#define fi first
#define se second
#define pii pair<int,int>
#define pli pair<ll,int>
#define pll pair<ll,ll>
#define mp(a, b) make_pair(a, b)
#define mt(a,b,c) make_tuple(a,b,c)
#define rank qwertyuio
#define next dfghjk
#define plus fsghsf
#define minus ytryr
#define pb push_back
using namespace std;
 
inline bool setmin(int &x, int y) { return (y < x) ? x = y, 1 : 0; }
inline bool setmax(int &x, int y) { return (y > x) ? x = y, 1 : 0; }
inline bool setmin(ll &x, ll y) { return (y < x) ? x = y, 1 : 0; }
inline bool setmax(ll &x, ll y) { return (y > x) ? x = y, 1 : 0; }
 
const int N = 100000;
const int inf = (int)1e9 + 1;
const ll big = (ll)1e18 + 1;
const int P = 239;
const int P1 = 31;
const int P2 = 57;
const int mod = (int)1e9 + 7;
const int mod1 = (int)1e9 + 9;
const int mod2 = 998244353;
const ld eps = 1e-12;
const double pi = atan2(0, -1);
const int ABC = 26;
#define ent '\n'
ll fac[100007]; 
 
bool sortbysec(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.second < b.second); 
} 
bool sortbyfirst(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.first > b.first); 
} 

ll power(ll x, ll y) 
{ 
    ll res = 1;      // Initialize result 
  
    x = x % mod;  // Update x if it is more than or 
                // equal to p 
  
    while (y > 0) 
    { 
        // If y is odd, multiply x with result 
        if (y & 1) 
            res = (res*x) % mod; 
  
        // y must be even now 
        y = y>>1; // y = y/2 
        x = (x*x) % mod; 
    } 
    return res; 
} 
ll modInverse(ll n) 
{ 
    return power(n, mod-2); 
} 
  

ll nCrModPFermat(ll n, ll r) 
{ 
   // Base case 
   if (r==0) 
      return 1; 
  
    return (fac[n]* modInverse(fac[r]) % mod * 
            modInverse(fac[n-r]) % mod) % mod; 
} 
//int saved[1000000007];
ll binarySearch(vector<ll> arr, ll l, ll r, ll x) 
{ 
    if (r>=l) 
    { 
        ll mid = l + (r - l)/2; 
        if (arr[mid] == x) 
            return mid; 
        if (arr[mid] > x) 
            return binarySearch(arr, l, mid-1, x); 
        return binarySearch(arr, mid+1, r, x); 
    } 
    return -1; 
} 
ll findPos(vector<ll> arr, ll key) 
{ 
    ll l = 0, h = 1; 
    ll val = arr[0]; 
  
    // Find h to do binary search 
    while (val < key) 
    { 
        l = h;        // store previous high 
        h = 2*h;      // double high index 
        val = arr[h]; // update new val 
    } 
 
    return binarySearch(arr, l, h, key); 
} 

vector<pii> seg;

vector<vector<int>> result(10005);
int main()
{
    //freopen("a.in", "r", stdin);
    //freopen("a.out", "w", stdout);<<" "
    /*int t;
    cin>>t;
    while(t--){
    

    
    }*/
    ll a,b;
    cin>>a>>b;
    if((a+b)%2){
        cout<<"IMPOSSIBLE";
    }
    else{
        cout<<(a+b)/2;
    }
    

       
    return 0;
}

Measured memory_bytes: 512