Problem:
Score: 100 points

Problem Statement

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.

Sample Input 1

2 16

Sample Output 1

9

|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.

Sample Input 2

0 3

Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

Sample Input 3

998244353 99824435

Sample Output 3

549034394

Solution in C++ (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int a,b,j,x,n,k,y;
    cin>>a>>b;
    if(a>=b)
        n=a;
    else
        n=b;
    for(int i=0;i<10000000;i++)
    {
        x=a-i;
        if(x<0)
            x=x*-1;
        y=b-i;
        if(y<0)
            y=y*-1;
        if(x==y)
        {
            cout<<i<<endl;
            return 0;

        }
    }
    cout<<"IMPOSSIBLE"<<endl;
    return 0;
}