#include<bits/stdc++.h>
using namespace std;
int main(){
  int a,b;cin>>a>>b;
  if(abs(b-a)&1)cout<<"IMPOSSIBLE"<<endl;
  else cout<<(a+b)/2<<endl;
  return 0;
}
### 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<bits/stdc++.h>
using namespace std;
int main(){
  int a,b;cin>>a>>b;
  if(abs(b-a)&1)cout<<"IMPOSSIBLE"<<endl;
  else cout<<(a+b)/2<<endl;
  return 0;
}
```