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 <bits/stdc++.h>
#define fastio ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define endl '\n'
#define display(v) for (auto x : v) cout << x << ", "
#define dbg(x) cout << (#x) << " = " << (x) << "\n"
#define dbgp(x) cout << (#x) << " = {" << (x.first) << ", " << (x.second) << "}\n";
#define print(x) cout << (#x) << " = [ ";display(x); cout << "]\n"
#define pb push_back
typedef long long int ll;
typedef unsigned long long int ull;
using namespace std;

int main()
{
	fastio;

    ll a, b;
    cin >> a >> b;
    ll c = a + b;
    if (c % 2)
    cout << "IMPOSSIBLE";
    else cout << c / 2;
    return 0;
}