#include<bits/stdc++.h>
using namespace std;

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

In CPP, your task is to solve 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(){
	int x,y;
	cin>>x>>y;
	if(x%2!=y%2){
		cout<<"IMPOSSIBLE";
	}
	else{
		cout<<(x+y)/2;
		
	}
	return 0;
}
```