#include<bits/stdc++.h>
using namespace std;
int main(){
	long long A,B;
	cin>>A>>B;
	if(A%2!=B%2)
	cout<<"IMPOSSIBLE";
	else 
	cout<<(A+B)/2;         
}
### Prompt

Generate 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>
using namespace std;
int main(){
	long long A,B;
	cin>>A>>B;
	if(A%2!=B%2)
	cout<<"IMPOSSIBLE";
	else 
	cout<<(A+B)/2;         
}
```