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: Runtime Error)

program main
    implicit none
    integer n, d
    integer km, i
    integer, allocatable :: m(:), y(:)
    read*,n
    allocate(m(n+1), y(n))

    read*, m
    read*, y

    km = 0

    do i=1,n
        d = y(i) - m(i)
        if(d>=0)then
            km = km + m(i)
            y(i) = y(i) - m(i)
            m(i) = 0

        else
            km = km + y(i)
            m(i) = m(i) - y(i)
            y(i) = 0
        endif

        if ( y(i)>0) then
            if(y(i) > m(i+1))then
                km = km + m(i+1) 
                y(i) = y(i) - m(i+1)
                m(i+1) = 0
            
            else
                km =  km + y(i)
                m(i+1) = m(i+1) - y(i)
                y(i) = 0
            endif
            
        end if
    enddo
    print'(i0)', km
  
end program main