#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
using namespace::std;
//struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;
__attribute__((constructor))
void init(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(15);
}
// random_device rd;
// mt19937 mt(rd());
// #include <boost/numeric/interval.hpp>
// #include <boost/numeric/interval/io.hpp>
// #include <boost/intrusive/rbtree.hpp>
// #include <boost/dynamic_bitset.hpp>
// #include <boost/geometry.hpp>
// #include <boost/geometry/geometries/linestring.hpp>
// #include <boost/geometry/geometries/polygon.hpp>
// #include <boost/geometry/geometries/point_xy.hpp>
// #include <boost/math/tools/minima.hpp>
// using boost::math::tools::brent_find_minima;
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// typedef mp::number<mp::cpp_dec_float<256>> cdouble;
// typedef mp::cpp_int cint;

#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
template<typename T=long long>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T=long long>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T=long long>using pqueue =__gnu_pbds::priority_queue<T, less<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
typedef vector<pair<lint,lint>> pvec;
typedef vector<vector<pair<lint,lint>>> pmat;
typedef vector<vector<vector<pair<lint,lint>>>> pmat3;
#define rep(i, n) for(lint i = 0; i < (lint)(n); i++)
#define irep(i) for(lint i = 0;; i++)
#define irep1(i) for(lint i = 1;; i++)
#define irep2(i) for(lint i = 2;; i++)
#define rrep(i, n) for(lint i = (lint)(n-1); i >-1; i--)
#define rrepi(i,a,b) for(lint i = (lint)(b-1); i >a-1; i--)
#define repi(i,a,b) for(lint i=lint(a);i<lint(b);i++)
#define rep2(i,a,b,c) for(lint i=lint(a);i>lint(b);i+=c)
#define all(x) (x).begin(),(x).end()
#define PI 3.141592653589793
#define dist(x1,y1,x2,y2) (pow(pow(x2-x1,2)+pow(y2-y1,2),0.5))
#define input(a,n) lint n;cin>>n;vector<lint>a(n);rep(i,n)cin>>a[i];
#define SUM(v) accumulate(all(v),0LL)
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define LINF 9223372036854775807
//#define MOD 998244353LL
#define MOD 1000000007LL
#define endl "\n"
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(int i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(int i=0;i<t.size();i++)output(t[i]);}
auto Y=[&](auto f){return[&](auto... args){return f(f, args...);};};
template<typename T=lint>T in(){return *istream_iterator<T>(cin);}
template<typename T=lint>inline T gcd(T a,T b){return b?gcd(b,a%b):a;}
template<typename T=lint>inline T lcm(T a,T b){return a/gcd(a,b)*b;}
template<typename T=lint>inline T minq(T a,T b){return min(a,b);}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
vector<int> dx={-1,1,0,0,1,1,-1,-1};
vector<int> dy={0,0,-1,1,1,-1,1,-1};

class FFT{
	using lint=long long;
	using C=complex<double>;
	lint n=1;
    public:
	vector<lint> mul(vector<lint> a,vector<lint> b){
		int size=a.size()+b.size()-1;
		while(n<size)n<<=1;
		auto ac=itoc(a,n);
		auto bc=itoc(b,n);
		auto ia=fft(ac,false);
		auto ib=fft(bc,false);
		vector<C> ires(n);
		for(int i=0;i<n;i++)ires[i]=ia[i]*ib[i];
		auto res=ctoi(fft(ires,true),size);
		return res;
	}
	private:
	vector<C> itoc(vector<lint> v,int size){
		vector<C> res(size);
		for(int i=0;i<min<int>(size,v.size());i++)res[i]=v[i];
		return res;
	}
	vector<lint> ctoi(vector<C> v,int size){
		vector<lint> res(size);
		for(int i=0;i<min<int>(size,v.size());i++)res[i]=round(v[i].real()/n);
		return res;
	}
	vector<C> fft(vector<C> v,bool inv){
		int n=v.size();
		assert((n&(n-1))==0);
		if(n==1)return v;
		vector<C> v1(n/2);
		vector<C> v2(n/2);
		for(int i=0;i<n/2;i++)v1[i]=v[i*2];
		for(int i=0;i<n/2;i++)v2[i]=v[i*2+1];
		auto iv1=fft(v1,inv);
		auto iv2=fft(v2,inv);
		vector<C> res(n);
		C zeta;
		for(int i=0;i<n;i++){
			if(inv)zeta=C(cos(2*M_PI*i/n),-sin(2*M_PI*i/n));
			else zeta=C(cos(2*M_PI*i/n),sin(2*M_PI*i/n));
			res[i]=iv1[i%(n/2)]+zeta*iv2[i%(n/2)];
		}
		return res;
	}
};

vec v;
mat a;
void f(lint p,lint n){
    for(lint t:a[n]){
        if(t==p)continue;
        v[t]+=v[n];
        f(n,t);
    }
}
int main(){
    lint a,b;
    cin>>a>>b;
    if((a+b)%2)cout<<"IMPOSSIBLE"<<endl;
    else cout<<(a+b)/2<<endl;
}

Measured memory_bytes: 256