/*|In The Name of Allah|*/
/* Dear GCC compiler:
*
* If my code doesn't compile, this doesn't mean it's wrong.
* It just mean that you don't understand what I want.
* Here's my safety pig, I hope my code compile and get accepted.
* _._ _..._ .-',     _.._(`))
*'-. `     '  /-._.-'    ',/
*   )         \            '.
*  / _    _    |             \
* |  a    a    /              |
* \   .-.                     ;
*  '-('' ).-'       ,'       ;
*     '-;           |      .'
*        \           \    /
*        | 7  .__  _.-\   \
*        | |  |  ``/  /`  /
*       /,_|  |   /,_/   /
*          /,_/      '`-'
*/
#include <bits/stdc++.h>
using namespace std;
long long power(long long x, long long y){
if (y == 0)
	return 1;
long long ret = power(x , y >> 1) %1000000007 ;
if (y % 2 == 0)
	return (ret * ret) % 1000000007;
else
	return (x * (ret * ret % 1000000007)) % 1000000007;
}

pair<int, int> norm(pair<int, int> p)
{
	if(p.first < 0)
	{
		p.first *= -1;
		p.second *= -1;
	}
	else if (p.first == 0 && p.second < 0)
	{
		p.second *= -1;
	}
	int d = __gcd(abs(p.first), abs(p.second));
	p.first /= d;
	p.second /= d;
	return p;
}

/*
	void dikstra(){
	for (int i=1;i<=n;i++)dst[i]=oo;
	clr(vis,0);
	clr(parent,-1);
	dst[1]=0;
	parent[1]=-1;
	s.insert( mp(0,1 ) );
	while(!s.empty()){

		pair<ll,ll> x=*s.begin();
		s.erase(s.begin());
		ll u=x.sc;
		ll d=x.fr;
		if (vis[u])continue;
		vis[u]=1;
		for (int i=0;i<v[u].size();i++){
			ll to=v[u][i].fr;
			ll cost=v[u][i].sc;
			if (vis[to]||d+cost>=dst[to])continue;
			dst[to]=d+cost;
			parent[to]=u;
			s.insert(mp(d+cost,to));
		}
	}




	vi v;
if (dst[n]==oo)return cout<< -1,0;

int nn=n;
	while(parent[nn]!=-1){
		v.push_back(nn);
		nn=parent[nn];
	}
		reverse(all(v));

	}


int parent[N], sz[N];
int n, m, components;

void init(){
  iota(parent, parent+n, 0);
  fill(sz, sz+n, 1);
  components = n;
}

int find(int u){
  if(parent[u] == u)  return u;
  return parent[u] = find(parent[u]);
}

void join(int u, int v){
  u = find(u), v = find(v);
  if(u == v)  return;
  if(sz[u] < sz[v]) swap(u, v);
  parent[v] = u;
  sz[u] += sz[v];
  --components;
}

*/
const double Eps =(1e-7);
int dcmp(double x,double y){ return fabs(x-y)<=Eps? 0: x<y ?-1:1; }
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<string> vs;
typedef vector<pair<int,int> > vii;
typedef map<int, int> mii;
typedef map<string, int> msi;
typedef pair<int, int> pii;
#define mii map <int, int>
#define mll map <ll, ll>
#define pii pair <int, int>
#define pll pair <ll, ll>
#define vi vector <int>
#define vd vector <double>
#define vll vector <ll>
#define sll set <ll>
#define spii set <pii>
#define vpii vector <pair <int,int> >
#define vpll vector <pair <long long,long long> >
#define vvi vector <vector <int> >
#define vvpii vector <vector <pii > >
#define sfi1(v) scanf("%d",&v)
#define sfi2(v1,v2) scanf("%d %d",&v1,&v2)
#define sfi3(v1,v2,v3) scanf("%d %d %d",&v1,&v2,&v3)
#define sfll1(v) scanf("%I64d",&v);
#define sfll2(v1,v2) scanf("%I64d %I64d",&v1,&v2)
#define sfll3(v1,v2,v3) scanf("%I64d %I64d %I64d",&v1,&v2,&v3)
#define min3(a,b,c)     min(a,min(b,c))
#define max3(a,b,c)     max(a,max(b,c))
#define min4(a,b,c,d)   min(a,min(b,min(c,d)))
#define max4(a,b,c,d)   max(a,max(b,max(c,d)))
#define el	'\n'
#define lp(i,n) for(int i = 0;i < int(n);i++)
#define lp1(i,n) for(int i = 1;i <= int(n);i++)
#define rlp(i, n) for (int i = (n) - 1; i >= 0; i--)
#define rng(i,a,b) for(int i = int(a);i <= int(b);i++)
#define mp(x,y)  	make_pair((x),(y))
#define pb(x)    	push_back((x))
#define all(v)  	((v).begin()),((v).end())
#define sz(x)  		((int) (x).size())
#define bits(a) 	__builtin_popcountll(a)
#define fr first
#define sc second
#define pii pair<int, int>
#define sortva(v) sort(all(v))
#define sortvd(v) sort(v.rbegin(),v.rend())
#define sortaa(a,n) sort(a,a+n)
#define sortad(a,n) sort(a,a+n),reverse(a,a+n)
#define SS stringstream
#define ll long long
#define clr(x, y) memset(x, y, sizeof(x))
#define foreach(it,x) for(__typeof__((x).begin()) it = (x).begin(); it != (x).end(); ++it)
#define read freopen("input.in", "rt", stdin);
#define write freopen("output.in", "wt", stdout);
#define fastIO cout << fixed << setprecision(0), ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);

int dx[] = { 0, 0, 1, -1, 1, -1, 1, -1 };
int dy[] = { 1, -1, 0, 0, -1, 1, 1, -1 };
int kx[] = { 2, 2, 1, -1, -2, -2, 1, -1 };
int ky[] = { 1, -1, 2, 2, 1, -1, -2, -2 };
ll gcd(ll a, ll b){if (a == 0)return b;return gcd(b % a, a);}
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }


int const N = 5e5+9, mod = 1e9+7 , oo = 1e9+7;

//------------------------------------------------------------------------------------


int a,b;
int main()
{
	cin>>a>>b;
	int x=abs(a+b);
	if (x%2==0)cout<< x/2;
	else cout<< "IMPOSSIBLE";








/*
*/
return 0;
}


Measured memory_bytes: 256