Problem:
Score: 100 points

Problem Statement

We have two distinct integers A and B.

Print the integer K such that |A - K| = |B - K|.

If such an integer does not exist, print IMPOSSIBLE instead.

Constraints

All values in input are integers.

0 \leq A,\ B \leq 10^9

A and B are distinct.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the integer K satisfying the condition.

If such an integer does not exist, print IMPOSSIBLE instead.

Sample Input 1

2 16

Sample Output 1

9

|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.

Sample Input 2

0 3

Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

Sample Input 3

998244353 99824435

Sample Output 3

549034394

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <algorithm>
#include <cmath>
#include <deque>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <map>
#include <numeric>
#include <queue>
#include <sstream>
#include <string>
#include <string.h>
#include <tuple>
#include <vector>

#define REP(i,x) for(int i{ 0 }; i < (int)(x); i++)
#define REPC(i,x) for(int i{ 0 }; i <= (int)(x); i++)
#define RREP(i,x) for(int i{ (int)(x) - 1 }; i >= 0 ;i--)
#define RREPC(i,x) for(int i{ (int)(x)}; i >= 0; i--)
#define REP1O(i,x) for(int i{ 1 }; i < (int)(x); i++)
#define REP1C(i,x) for(int i{ 1 }; i <= (int)(x); i++)
#define REPIT(i,x) for(auto i{(x).begin()}; i != (x).end(); i++) 

#define PB push_back
#define MP make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(),(x).end()

using namespace std;

typedef int64_t ll;
typedef double dbl;
typedef vector<bool> Vb;
typedef vector<char> Vc;
typedef vector<double> Vd;
typedef vector<int> Vi;
typedef vector<ll> Vl;
typedef vector<string> Vs;
typedef vector<vector<int>> VVi;
typedef vector<vector<ll>> VVl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ll, int> pli;

template<class T> inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
inline void swap(ll& a, ll& b) { a ^= b; b ^= a; a ^= b; }
inline void swap(int& a, int& b) { a ^= b; b ^= a; a ^= b; }
inline void YES() { cout << "YES" << endl; } inline void Yes() { cout << "Yes" << endl; }
inline void NO() { cout << "NO" << endl; } inline void No() { cout << "No" << endl; }

const int inf = 1 << 30;
const ll linf = 1LL << 60;
const int MOD = 1000000007;


int main()
{
	ll a, b;
	cin >> a >> b;

	if (a > b) swap(a, b);

	if ((b - a) & 1)
		cout << "IMPOSSIBLE" << endl;
	else
		cout << (a + b) / 2 << endl;

}