//Generated at 2019-07-27 20:02:38 UTC+8
//Created by Alphagocc
#if __cplusplus < 201103L
#define ___TR1(x) <tr1/x>
#else
#define ___TR1(x) <x>
#endif
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <climits>
#include <clocale>
#include <cmath>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include ___TR1(cstdbool)
#include ___TR1(cstdint)
#include ___TR1(ctgmath)
#include ___TR1(cfenv)
#include ___TR1(cinttypes)
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cstdalign>
#else
#include <tr1/complex>
#endif
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include ___TR1(array)
#include ___TR1(random)
#include ___TR1(tuple)
#include ___TR1(unordered_map)
#include ___TR1(unordered_set)
#if __cplusplus >= 201103L
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <type_traits>
#include <typeindex>
#endif
#ifdef USE_PBDS
#include <ext/pb_ds/assoc_container.hpp>
#endif
// struct IO
struct IO
{
    bool goodReadBit;
    IO() { goodReadBit = 1; space = ' ';}
    template <typename T>
    void __RI(T &x)
    {
        int ch = getchar(), neg = 1;
        x = 0;
        for (; !(isdigit(ch) || ch == '-' || ch == EOF); ch = getchar())
            ;
        if (ch == EOF)
        {
            goodReadBit = 0;

            return;
        }
        if (ch == '-') neg = -1, ch = getchar();
        for (; isdigit(ch); ch = getchar()) x = x * 10 + ch - 48;
        x *= neg;
    }
    void R(int &x) { __RI(x); }
    void R(int64_t &x) { __RI(x); }
    template <typename T1, typename T2>
    void R(std::pair<T1, T2> &x)
    {
        R(x.first);
        R(x.second);
    }
    template <typename T>
    void R(std::vector<T> &x)
    {
        for (typename std::vector<T>::iterator it = x.begin(); it != x.end(); ++it) { R(*it); }
    }
    template <typename T>
    void R(T &x)
    {
        std::cin >> x;
        if (!std::ios::goodbit) goodReadBit = 0;
    }
#if __cplusplus >= 201103L
    template <typename T, typename... Args>
    void R(T &x, Args &... args)
    {
        R(x), R(args...);
    }
#endif
    bool goodRead() { return goodReadBit; }
    char space;
    template <typename T>
    void __WI(T x)
    {
        if (x == 0) putchar('0');
        if (x < 0) putchar('-'), x = -x;
        char ch[ 64 ];
        int  pos = 0;
        while (x) ch[ pos++ ] = x % 10 + 48, x /= 10;
        while (pos) putchar(ch[ --pos ]);
    }
    void W() {}
    void W(const int &x) { __WI(x); }
    void W(const int64_t &x) { __WI(x); }
    void W(const double &x) { printf("%lf", static_cast<double>(x)); }
    void W(const char &x) { putchar(x); }
    void W(const std::string &x) { printf("%s", x.c_str()); }
    template <typename T1, typename T2>
    void W(const std::pair<T1, T2> &x)
    {
        W(x.first);
        W(space);
        W(x.second);
    }
    template <typename T>
    void W(const std::vector<T> &x)
    {
        for (typename std::vector<T>::const_iterator it = x.begin(); it != x.end(); it++)
        {
            W(*it);
            W(space);
        }
        W('\n');
    }
    template <typename T>
    void W(const T &x)
    {
        std::cout << x;
    }
    void WL() { W('\n'); }
    template <typename T>
    void WL(const T &x)
    {
        W(x), W('\n');
    }
    void WS() { W(' '); }
    template <typename T>
    void WS(const T &x)
    {
        W(x), W(' ');
    }
#if __cplusplus >= 201103L
    template <typename T, typename... Args>
    void W(const T &x, const Args &... args)
    {
        W(x), W(space), W(args...);
    }
    template <typename... Args>
    void WL(const Args &... args)
    {
        W(args...), W('\n');
    }
#endif
    template <typename T>
    IO &operator>>(T &x)
    {
        R(x);
        return (*this);
    }
    template <typename T>
    IO &operator<<(const T &x)
    {
        W(x);
        return (*this);
    }
} io; // struct IO
namespace Alphagocc
{
#if __cplusplus >= 201103L
decltype(std::chrono::high_resolution_clock::now()) tStart;
};
void      TimeBegin() { Alphagocc::tStart = std::chrono::high_resolution_clock::now(); }
long long TimeEnd()
{
    return (std::chrono::high_resolution_clock::now() - Alphagocc::tStart).count();
#endif
} // namespace Alphagocc
#if __cplusplus >= 201103L
std::random_device randomDevice;
std::mt19937       randomGenerator(randomDevice());
#else
std::tr1::random_device randomDevice;
std::tr1::mt19937       randomGenerator(randomDevice());
#endif
using namespace std;
#if __cplusplus < 201103L
using namespace tr1;
#endif
#define REP(i, x, y) for (int i = x; i < y; i++)

const int8_t  INFC = 0x3f;
const int16_t INFS = 0x3f3f;
const int32_t INF  = 0x3f3f3f3f;
const int64_t INFL = 0x3f3f3f3f3f3f3f3fLL;

int main()
{
    int a,b;
    io>>a>>b;
    if ((a&1)!=(b&1))
    {
        io.WL("IMPOSSIBLE");
    }else
    {
        io<<(0LL+a+b)/2;
    }
    return 0;
}

Measured memory_bytes: 256