import std.stdio;
import std.conv;
import std.array;
import std.math;

void main()
{
    auto reader = readln.split;
    int A = reader[0].to!int;
    int B = reader[1].to!int;

    int diff = abs(A - B);
    if (diff % 2 == 0){
        writeln((A + B) / 2);
    } else {
        writeln("IMPOSSIBLE");
    }
}

