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 <stdlib.h>
#include <math.h>
#include <string.h>


int main(void) {
int A,B;
scanf("%d %d",&A,&B);
int max =0;
if (A>B) max=A;
else max =B;
for (int i=0;i<max;i++){
    if (abs(A-i)==abs(B-i)){printf("%d",i);
                             break;          }
 else if ((abs(A-i)!=abs(B-i)) && i==max-1) {printf("IMPOSSIBLE");}

}
return EXIT_SUCCESS;
}