#include <iostream>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <fstream>
#include <bitset>
#include <queue>
#include <stack>
#include <deque>
#include <time.h>
#include <complex>
 
using std::cin;
using std::cout;
using std::cerr;
using std::endl;
 
using std::map;
using std::set;
using std::bitset;
using std::vector;
using std::string;
using std::multimap;
using std::multiset;
using std::deque;
using std::queue;
using std::stack;
using std::pair;
using std::iterator;
using std::complex;
 
using std::sort;
using std::stable_sort;
using std::reverse;
using std::max_element;
using std::min_element;
using std::unique;
using std::ios_base;
using std::swap;
using std::fill;
 
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef string S;
typedef complex<double> cd;
 
ll min(ll a, ll b)
{
    if (a < b)
        return a;
    return b;
}
 
ll max(ll a, ll b)
{
    if (a > b)
        return a;
    return b;
}
 
namespace MySpace{
    struct Small_Segment_Tree{
        vector<int> a;
        int l, r;
 
        private:
        int __summ(int L, int R, int l, int r, int v)
        {
            if (L <= l && r <= R)
                return a[v];
            if (R <= l || r <= L)
                return 0;
            int m = (l + r) / 2;
            return __summ(L, R, l, m, 2 * v + 1) + __summ(L, R, m, r, 2 * v + 2);
        }
 
        void __set(int P, int l, int r, int v, int V)
        {
            if (l + 1 == r)
            {
                a[v] = V;
                return;
            }
            int m = (l + r) / 2;
            if (P < m)
            {
                __set(P, l, m, 2 * v + 1, V);
                a[v] = a[2 * v + 1] + a[2 * v + 2];
            }
            else
            {
                __set(P, m, r, 2 * v + 2, V);
                a[v] = a[2 * v + 1] + a[2 * v + 2];
            }
        }
 
        public:
        int Summ(int L, int R)
        {
            if (R < L)
                return 0;
            return __summ(L, R + 1, l, r, 0);
        }
 
        void Set(int P, int V)
        {
            __set(P, l, r, 0, V);
        }
 
        Small_Segment_Tree(int _n)
        {
            a.resize(4 * _n);
            l = 0, r = _n;
        }
    };
    long long inq(long long x, long long q, long long MOD)
    {
        if (q == 0)
            return 1;
        long long l = inq(x, q / 2, MOD);
        if (q % 2)
            return l * l % MOD * x % MOD;
        return l * l % MOD;
    }
};
 
 
using namespace MySpace;
 
#define F(i, n) for (int (i) = 0; (i) != (n); (i)++)
#define fi first
#define se second
#define re return
#define all(x) (x).begin(), (x).end()
#define int long long
 
void YN(int val)
{
    if (val >= 1)
    {
        printf("YES\n");
    }
    else
    {
        printf("NO\n");
    }
}
 
//const ll MOD = 998244353;
const ll MOD = 1e9 + 7;

ll a, b;

signed main()
{
    ios_base::sync_with_stdio();
    cin.tie(0);
    cout.tie(0);
    cin >> a >> b;
    if (b % 2 != a % 2)
    {
        cout << "IMPOSSIBLE";
        return 0;
    }
    if (b < a)
        swap(a, b);
    cout << (a + b) / 2;
}

Measured memory_bytes: 256