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 Fortran (gfortran v4.8.4) (judge verdict: Accepted)

program kadai
implicit none
integer  ::  a,b,i,k


read(*,*) a,b

if (a<b) then
    k=b
else if (a>b) then
    k=a
end if

do while(k>0)
    if (abs(a-k)==abs(b-k)) then
        write(*,*) k
        stop
    end if
    k=k-1
end do

write(*,*) "IMPOSSIBLE"


stop

end program kadai