#include <iostream>

int main(){
	int A,B;
  	scanf("%d %d",&A,&B);
  if((A+B)%2==0){
  	printf("%d",(A+B)/2);
  }
  else{
  	printf("IMPOSSIBLE");
  }
}
### Prompt

Develop a solution in CPP to the problem described below:

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>

int main(){
	int A,B;
  	scanf("%d %d",&A,&B);
  if((A+B)%2==0){
  	printf("%d",(A+B)/2);
  }
  else{
  	printf("IMPOSSIBLE");
  }
}
```