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 Python (3.4.3) (judge verdict: Runtime Error)

import numpy as np

mod = 10**9+7
p = list(input())
# p = list("?"*10**4)

# アルゴリズム
dp = np.zeros(13, dtype=int)
dp[0] = 1
mul = 1
for key in reversed(p):
    new_dp = np.zeros(13, dtype=int)
    if key == "?":
        for i in range(10):
            for j in range(13):
                new_dp[(j + i * mul) % 13] += dp[j]
                new_dp[(j + i * mul) % 13] %= mod
    else:
        
        for j in range(13):
            new_dp[(j + int(key) * mul) % 13] += dp[j]
            new_dp[(j + int(key) * mul) % 13] %= mod
    dp = new_dp.copy()
    mul *= 10

print(dp[5])