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 PyPy3 (2.4.0) (judge verdict: Runtime Error)

# -*- coding: utf-8 -*-

S = input()

MOD =[1,0,0,0,0,0,0,0,0,0,0,0,0]
NMOD =[0,0,0,0,0,0,0,0,0,0,0,0,0]

R10 = range(10)
R13 = range(13)

for i,D in enumerate(S):
    C = -1
    if D == '?':
        for N in R10:
            for M in R13:
                NM = (M * 10 + N) % 13
                NMOD[NM] += MOD[M]
            #end for
        #end for
    else:
        C = int(D)
        for M in R13:
            NM = (M * 10 + C) % 13
            NMOD[NM] += MOD[M]
        #end for
    #end if
    for M in R13:
        MOD[M] = NMOD[M]%1000000007
        NMOD[M] = 0
    #end for
#end for

print(MOD[5])