/***************************************
Author : Susanka Majumder ( bingobong )
***************************************/

#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif

#include<bits/stdc++.h>
using namespace std;

/* Template file for Online Algorithmic Competitions */
/* Typedefs */
    /* Basic types */
    typedef long long           ll;
    typedef long double         ld;
    typedef unsigned long long ull;
    /* STL containers */
    typedef vector <int>    vi;
    typedef vector <ll>     vll;
    typedef pair <int, int> pii;
    typedef pair <ll, ll>   pll;
    typedef vector < pii >  vpii;
    typedef vector < pll >  vpll;
    typedef vector <string> vs;
    typedef vector < vi >   vvi;
    typedef vector < vll >  vvll;
    typedef vector < vpii > vvpii;
    typedef set <int>       si;
/* Macros */
    /* Loops */
    #define fl(i, a, b)     for(int i(a); i <= (b); i ++)
    #define rep(i, n)       fl(i, 1, (n))
    #define loop(i, n)      fl(i, 0, (n) - 1)
    #define rfl(i, a, b)    for(int i(a); i >= (b); i --)
    #define rrep(i, n)      rfl(i, (n), 1)
    /* Algorithmic functions */
    #define pow2(x) ((x)*(x))
    #define mod(x, m) ((((x) % (m)) + (m)) % (m))
    #define max3(a, b, c) max(a, max(b, c))
    #define min3(a, b, c) min(a, min(b, c))
    #define srt(v)          sort((v).begin(), (v).end())
    /* STL container methods */
    #define pb              push_back
    #define mp              make_pair
    #define eb              emplace_back
    /* String methods */
    #define dig(i)          (s[i] - '0')
    #define slen(s)         s.length()
    /* Shorthand notations */
    #define fr              first
    #define sc              second
    #define re              return 
    #define sz(x)           ((int) (x).size())
    #define all(x)          (x).begin(), (x).end()
    #define sqr(x)          ((x) * (x))
    #define fill(x, y)      memset(x, y, sizeof(x))
    #define clr(a)          fill(a, 0)
    #define endl            '\n'
    /* Mathematical */
    #define IINF            0x3f3f3f3f
    #define LLINF           1000111000111000111LL
    #define PI              3.14159265358979323
    #define NIL             -1
    /* Debugging purpose */
    #define trace1(x)                cerr << #x << ": " << x << endl
    #define trace2(x, y)             cerr << #x << ": " << x << " | " << #y << ": " << y << endl
    #define trace3(x, y, z)          cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": " << z << endl
    /* Fast Input Output */
    #define FAST_IO                  ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
/* Constants */
    const ll MOD = 1000000007LL;
    const ll MAX = 100010LL;
/* Templates */
template<class T> T abs(T x) { re x > 0 ? x : -x; }
template<typename T> T gcd(T a, T b){ if(b == 0) return a; return gcd(b, a % b); }
template<typename T> T power(T x, T y, ll m = MOD){T ans = 1; x %= m; while(y > 0){ if(y & 1LL) ans = (ans * x)%m; y >>= 1LL; x = (x*x)%m; } return ans%m; }
/* additional*/
#define tr(c,i) for(typeof((c).begin() i = (c).begin(); i != (c).end(); i++)
#define present(c,x) ((c).find(x) != (c).end())
#define cpresent(c,x) (find(all(c),x) != (c).end())
//double inf = 1.0/0.0;

int main(){

   /* #ifndef ONLINE_JUDGE
    freopen("C:\\Users\\LENOVO\\Desktop\\input.txt","r",stdin);
    freopen("C:\\Users\\LENOVO\\Desktop\\output.txt","w",stdout);
    freopen("C:\\Users\\LENOVO\\Desktop\\error.txt","w",stderr);
    #endif
    */
    
    FAST_IO;
    ll a,b;
    cin>>a>>b;
    if((a+b)&1)
    	cout<<"IMPOSSIBLE"<<'\n';
    else
    	cout<<(a+b)/2<<'\n';
   /* #ifndef ONLINE_JUDGE
    cout<<"\nTime Elapsed: " << 1.0*clock() / CLOCKS_PER_SEC << " sec\n";
    #endif*/
    return 0;
}

Measured memory_bytes: 256