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)

a=int(input())
b=input().split(" ")
sequence=[]
for i in range(0,a):
    sequence.append(int(b[i]))
error=[]
for q in range(0,a-1):
    if sequence[q]-sequence[q+1]>0:
        error.append(q)
        error.append(q+1)

if len(error)==0:
    print("YES")

elif len(error)==2:
    if a==2:
        print("YES")
    elif error[0]==0:
        if sequence[error[0]+2]-sequence[error[0]]>0:
            print("YES")
        else:
            print("NO")
    elif error[1]==a:
        if sequence[error[1]]-sequence[error[1]-2]>0:
            print("YES")
        else:
            print("NO")
    else:
        if  sequence[error[1]]-sequence[error[1]-2]>0 and sequence[error[0]+2]-sequence[error[0]]>0:
            print("YES")
        else:
            print("NO")

elif len(error)==4:
    if error[1]==error[2]:
        if sequence[error[0]]>sequence[error[3]]:
            if error[0]==0 and error[3]==a-1:
                if sequence[error[0]]-sequence[error[3]-1]>0 and sequence[error[0]+1]-sequence[error[3]]>0:
                    print("YES")
                else:
                    print("NO")
            elif error[0]==0:
                if sequence[error[0]+3]-sequence[error[0]]>0:
                    print("YES")
                else:
                    print("NO")
            elif error[1]==a:
                if sequence[error[3]]-sequence[error[3]-3]>0:
                    print("YES")
                else:
                    print("NO")
            else:
                if  sequence[error[3]]-sequence[error[3]-3]>0 and sequence[error[0]+3]-sequence[error[0]]>0:
                    print("YES")
                else:
                    print("NO")
    elif sequence[error[0]]-sequence[error[2]]>0 and sequence[error[1]]-sequence[error[3]]>0:
        if error[0]==0 and error[3]==a-1:
            if sequence[error[0]]-sequence[error[3]-1]>0 and sequence[error[0]+1]-sequence[error[3]]>0:
                print("YES")
            else:
                print("NO")
        elif error[0]==0:
            if sequence[error[3]+1]-sequence[error[0]]>0:
                print("YES")
            else:
                print("NO")
        elif error[3]==a:
            if sequence[error[3]]-sequence[error[0]-1]>0:
                print("YES")
            else:
                print("NO")
        elif sequence[error[3]]-sequence[error[0]-1]>0 and sequence[error[3]+1]-sequence[error[0]]>0:
            print("YES")
        else:
            print("NO")
    else:
        print("NO")

else:
    print("NO")