#include <bits/stdc++.h>

int main()
{
	int A, B;
	scanf("%d%d", &A, &B);
	if ((A + B) & 1) puts("IMPOSSIBLE");
	else printf("%d\n", (A + B) / 2);

	return 0;
}
### Prompt

Please formulate 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 <bits/stdc++.h>

int main()
{
	int A, B;
	scanf("%d%d", &A, &B);
	if ((A + B) & 1) puts("IMPOSSIBLE");
	else printf("%d\n", (A + B) / 2);

	return 0;
}
```