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 <string.h>
#include <math.h>
int compare_int(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
//    return *(long long*)a - *(long long*)b;
}
//  qsort(A, b, sizeof(int), compare_int);

long long  factorial(long long n) {
    if (n > 0) {
        return n * factorial(n - 1);
    } else {
        return 1;
    }
}

int min(int a, int b) {
    if (a > b)
        return b;
    else
        return a;
}

int distance(int a, int b) {
  return abs(a-b)*abs(a-b);
}

//int ()



int main()
{
//  int a ,b, c, d, e, X, Y;
  long long  a, b, c, d, e, X, Y;
  //  double  c;
  int  A[200003]={};
//  long long  A[100003]={};
//  long long  B[100003]={};
  int  C[100003][11]={};

  int  flg=0;

//  long long  a, b, c, d, X, Y;
  char s[101][101]={};
  char t[101]={};
//  int N, M;
//  int A[1000000001]={};
//  long long  C[101]={};
  int i,j,k;
  long long p,q;
//  int flag=0;
//  int tmp=0;
  char tmp[101]={};
//  long long min=10000000000;
//  long long min=100000001;
  int len=0;
  int l=0;
  int r=1000001;
  int cnt=0;
  int num=0;
  int num1=0;
  int num2=0;
  long long ans=1;
  int max=0;
  int sum=0;
  int addr=0;
  int ptn=0;
//  bool on_off[100]={};
  
//  char alp[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

  //  a=0;
//  scanf("%lld %lld", &a, &b);
  scanf("%lld%lld", &a,&b);
  if((a+b)%2==0){
    if(a>b) printf("%lld\n", llabs(a-b)/2+b);
    else if(a<=b) printf("%lld\n", llabs(a-b)/2+a);
  }
  else   printf("IMPOSSIBLE\n");
  

    
//    printf("%s %d %d\n", s[i],C[i][0],C[i][1]);
   // printf("%d\n", C[i][1]);
/*  for(i=0;i<addr;i++){
//  scanf("%lld%lld", &B[0][i],&B[1][i]);
//    ans=ans+A[i];
    printf("%lld ", A[i]);
  }
*/
//    printf("%d ", cnt);
   

return 0;
}