#include <stdio.h>

int main(void)
{
	int a, b;
  
	scanf("%d %d" ,&a, &b);

	if ((a + b) % 2)
		puts("IMPOSSIBLE");
	else
		printf("%d" ,(a + b) / 2);
	return 0;
}
### Prompt

Construct a Cpp code solution to the problem outlined:

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 <stdio.h>

int main(void)
{
	int a, b;
  
	scanf("%d %d" ,&a, &b);

	if ((a + b) % 2)
		puts("IMPOSSIBLE");
	else
		printf("%d" ,(a + b) / 2);
	return 0;
}
```