import std.stdio, std.string, std.conv, std.array, std.algorithm, std.range, std.typecons, std.numeric, std.math;

void main() {
    long a, b;
    readf("%d %d\n", &a, &b);
    if (a > b) {
        long tmp = a;
        a = b;
        b = tmp;
    }
    if ((b - a) % 2 == 0) {
        writeln((b - a) / 2 + a);
    } else {
        writeln("IMPOSSIBLE");
    }
}
