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,big,small,x=0;
    scanf("%d%d",&a,&b);
    if((0<=a<=pow(10,9))&&(0<=b<=pow(10,9)))
    if(a>b){big=a;small=b;}
    else {big=b;small=a;}
    for(;big>=small;big--){
        if(abs(a-big)==abs(b-big))x=big;
    }
    if(x==0)printf("IMPOSSIBLE");
    else printf("%d",x);

    return 0;
}