#include <iostream>
using namespace std;

int main() {
	int A, B;
	cin >> A >> B;
	if ((A + B) % 2 == 0) cout << (A + B) / 2;
	else cout << "IMPOSSIBLE";
}
### Prompt

Your task is to create a Cpp solution to the following problem:

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.

Examples

Input

2 16


Output

9


Input

0 3


Output

IMPOSSIBLE


Input

998244353 99824435


Output

549034394

### Response

```cpp
#include <iostream>
using namespace std;

int main() {
	int A, B;
	cin >> A >> B;
	if ((A + B) % 2 == 0) cout << (A + B) / 2;
	else cout << "IMPOSSIBLE";
}
```