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: Accepted)

#include<stdio.h>
#include<math.h>
int main()
{
    int A,B,i,max,c = 1,x,y;
    scanf("%d%d",&A,&B);
    if(A > B)
    {
        max = A;
    }
    else
    {
        max = B;
    }
    for(i = 1 ; i <= max ; i = i + 1)
    {
        x = abs(A - i);
        y = abs(B - i);
        if(x == y)
        {
            c = 5;
            break;
        }
    }
    if(c == 5)
    {
        printf("%d\n",i);
    }
    else
    {
        printf("IMPOSSIBLE\n");
    }
    return 0;
}