#include <cstdlib>
#include <iostream>
#include <string>
#include <time.h>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <sstream>
#include <iomanip>
#include <vector>
#include <functional>
#include <numeric>
#include <cassert>
#include <algorithm>
#include <functional>
#include <tuple>
#include <bitset>
#include <queue>
#include <map>
//#include "bits/stdc++.h"
using namespace std;
//using namespace std::vector;
#define rep(i,a,b) for(ll i=(a);i<(b);++i)
//#define M_PI 3.1415926535
#define huge 1000000007
typedef long long int ll;
ll gcd(ll a, ll b);
ll lcm(ll x, ll y);
ll bur(ll N, ll X);
long long comb(int n, int r);
bool IsPrime(int num);
bool dp[110][10010];
class UnionFind;

//UF木
class UnionFind {
public:
	//親
	vector<int> Parent;

	//初期状態
	UnionFind(int N) {
		Parent = vector<int>(N, -1);
	}

	int root(int A) {
		if (Parent[A] < 0) return A;
		return Parent[A] = root(Parent[A]);
	}

	//サイズ確認
	int size(int A) {
		return -Parent[root(A)];
	}

	int connect(int A, int B) {
		//root同氏をくっつける
		A = root(A);
		B = root(B);

		//結合済み
		if (A == B) {
			return false;
		}
		//大きいほうに小さいほうをくっつける
		if (size(A) < size(B)) swap(A, B);

		//Aのサイズ更新
		Parent[A] += Parent[B];
		//Bの親をAに
		Parent[B] = A;

		return true;
	}

};

int main(void) {

	ll N = 0;
	ll A = 0;
	ll B = 0;
	ll C = 0;
	ll D = 0;
	ll E = 0;
	ll W = 0;
	ll H = 0;
	//ll T = 0;
	//ll M = 0;
	//ll h[400][400] = {};
	ll b[9] = { 1,7,4,5,3,2,9,6,8 };//順番の配列
	//ll b2[9] = { 2,3,4,5,5,5,6,6,7 };//順番の配列

									 //ll aa[20][20] = {};//sortaf
	//ll c2[9] = {};

	ll d[10] = {};//要素数

				  //ll v[3] = {};
				  //ll *c, *s, *n;
	string S, T, U[52];
	char fi[50], fv[50];

	ll count = 0;
	double count2 = 0;

	ll min = 3000;
	ll max = 1;
	ll dist1 = 0;
	ll dist2 = 0;
	ll dist3 = 0;
	ll now = 0;
	int tmax = 10000;
	int tmax2 = 0;
	//int flag = 0;
	//char temp, head, state;
	//int is = 0;
	bool start = false;

	//char p;

	float all = 0,all2 = 0,all3=0;
	ll amr = 0;
	ll first = 0;
	ll second = 0;
	ll keep = 0;
	//ll head = 0;
	//ll tail = 0;
	//ll lgh = 10000;
	//ll state = 0;
	char col = NULL;

	//ll aa[3] = {10000,10000,10000}, bb[3] = { 10000,10000,10000 }, cc[3] = { 10000,10000,10000 };
	//ll an[3] = { 10000,10000,10000 }, bn[3] = { 10000,10000,10000 }, cn[3] = { 10000,10000,10000 };

	cin >> A >> B;
	//vector<ll> mo(A+1);
	vector<ll> dam(N);
	//vector<ll> dam2(N);
	
	/*rep(i, 0, N) {
		cin >> dam[i];
	}*/
	
	if (A > B) {
		swap(A, B);

	}

	if (abs(A - B) % 2 != 0) {
		cout << "IMPOSSIBLE" << endl;
	}
	else {
		cout << A + (B - A) / 2 << endl;
	}



	//cout << count << endl;

	return 0;

}

ll bur(ll N, ll X) {
	ll pan = 1;
	ll pat = 1;
	ll burg = 0;



	if (X > burg) {
		if (N == 0)
			return pat;


		burg += pan;
		burg += bur(N - 1, X);
		burg += pat;
		burg += bur(N - 1, X);
		burg += pan;


	}

	return burg;

}

ll gcd(ll a, ll b) {

	/* 自然数 a > b を確認・入替 */
	if (a<b) {
		ll tmp = a;
		a = b;
		b = tmp;
	}
	ll x = b;
	ll y = a;
	/* ユークリッドの互除法 */
	ll r = a % b;
	while (r != 0) {
		a = b;
		b = r;
		r = a % b;
	}


	return y / b*x;
}

bool IsPrime(int num)
{
	if (num < 2) return false;
	else if (num == 2) return true;
	else if (num % 2 == 0) return false; // 偶数はあらかじめ除く

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// 素数ではない
			return false;
		}
	}

	// 素数である
	return true;
}

// 最大公約数を求める関数
ll lcm(ll x, ll y)
{
	if (x == 0 || y == 0)  // 引数チェック
	{
		//fprintf(stderr, "引数エラーです。\n");
		return 0;
	}

	return (x * y / gcd(x, y));
}

//combination
long long comb(int n, int r) {
	std::vector<std::vector<long long>> v(n + 1, std::vector<long long>(n + 1, 0));
	for (int i = 0; i < v.size(); i++) {
		v[i][0] = 1;
		v[i][i] = 1;
	}
	for (int j = 1; j < v.size(); j++) {
		for (int k = 1; k < j; k++) {
			v[j][k] = (v[j - 1][k - 1] + v[j - 1][k]);
		}
	}
	return v[n][r];
}

Measured memory_bytes: 256