import core.bitop, std.bitmanip;
import core.checkedint;
import std.algorithm, std.functional;
import std.array, std.container;
import std.bigint;
import std.conv;
import std.datetime.stopwatch;
import std.math, std.numeric;
import std.range, std.range.interfaces;
import std.stdio, std.string;
import std.typecons;

void main()
{
    int a, b;
    readf("%s %s", &a, &b);
    readln;
    
    int k = (a + b) / 2;
    if (abs(a - k) != abs(b - k)) {
        writeln("IMPOSSIBLE");
        return;
    }
    
    k.writeln;
}